…Android manifest (take 2)" (#182522)

### Overview
Re-lands #181632 which TLDR does
the following:

**Same old same old changes from original PR**
- Refactors all engine flags recognized by the Flutter Android embedding
(specifically those previously recognized by
[`FlutterShellArgs`](https://github.com/flutter/flutter/blob/4f5478cce38d837e14b7a032a12500d3fc0f1310/engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/FlutterShellArgs.java)
and all of those used to initialize the engine in `FlutterLoader`) into
a new class `FlutterEngineFlags` (`FlutterShellArgs` is kept for now to
maintain backwards compatibility; see
#182153 for details)
- Adds the ability to set engine flags (that are recognized by
`FlutterEngineFlags`) via the manifest
- Adds warnings for developers that set engine flags via `Intent`s as
this will be disabled as part of
#180686.
- Disallows debug/test only flags in release mode

**New changes/fixes**

> [!NOTE]
> This re-land was required because when previously submitted, it caused
b/483299339. I have confirmed that this bug has been fixed in this PR;
see go/flutter-android-setting-flags-in-manifest-perf-comparison for A/B
test results with the previous version of this PR
(#181632) and commit
d6d4a07
of this PR.

- [Critical change] To fix the bug caught by b/483299339,
`--merged-platform-ui-thread` flag is added to `FlutterEngineFlag`s
- [Behavior change, not critical] To avoid more bugs like b/483299339,
`FlutterLoader` allows unrecognized flags specified by the command line
or `Intent` extras to be used to initialize the engine for now (same
behavior as before my original change but I plan to secure this in the
future; see #182557)
- [Small refactor] To speed up manifest argument processing time,
`FlutterLoader` loops through all recognized `FlutterEngineFlag`s to
check if they are present in an application's manifest versus looping
through all metadata in an application's manifest and looking for
recognized `FlutterEngineFlag`s (small improvement for apps with large
manifests)
- [Small refactor] To speed up command line argument processing time,
`FlutterLoader` processes the `--aot-shared-library-name` flag in the
same loop as the other flags, reducing the number of loops
`ensureInitializationComplete` has to do (minimal improvement perf-wise
but kept it in because it's more readable to me)

### Full description
(Mostly copied from #181632)

> [!NOTE]  
> This PR is based on conversation & feedback on
go/flutter-android-harden-engine-shell-arguments.

Adds a mechanism for setting Android engine flags via the manifest. If a
flag is specified on the command line and in manifest metadata, the
value specified on the command line will take precedence. Documentation
is added on this mechanism

Additionally, this PR removes the exposure of`--cache-sksl` command line
flag as per
[https://github.com/flutter/flutter/issues/140310#issuecomment-2708459007](https://www.google.com/url?q=https://github.com/flutter/flutter/issues/140310%23issuecomment-2708459007&sa=D&source=docs&ust=1761156167162464&usg=AOvVaw3a8ubXTtv3apknY2-P9dKe).

Additionally, this PR adds documentation for the only two supported ways
of setting engine flags moving forward -- via the command line or
manifest. The `Intent` mechanism will be removed when
#180686 is completed (intended
to be a follow up to this PR).

As the unit tests in this PR only cover setting flags via manifest in
debug mode, I will follow up this PR with an integration test to test
that flags are appropriately respected/ignored in release mode. See
#178383 for my currently working
but WIP draft.

Part of #172553.

# Follow up work:

- [ ] **Migrate flags all un-migrated flags `FlutterEngineFlags`**
#182852
- [ ] **Add integration test for this new added mechanism.** This will
land as an immediate follow-up to this PR. WIP in
#182241.
- [ ] **Remove support for setting shell arguments via `Intent`.** This
task will be a follow up to this work + the integration test landing,
and will complete work for
#172553. See
#180686 for details.
- [ ] **Only allow known engine flags to be passed to the engine.** This
is more of a P2 but would be a nice additional security feature; see
#182557.

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- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

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