…run (flutter#139614)

## Summary
Fixes flutter#139180, where `flutter create` could crash if the `java` binary the tool found cannot be run.

## Context

At startup, the tool searches for a Java installation[^1]. Unless the located installation is from [an Android Studio installation](https://github.com/flutter/flutter/blob/e1967ecabf014bf93d1731fde6a6547b06ca9c33/packages/flutter_tools/lib/src/android/android_studio.dart#L163), the tool does not verify that the binary is runnable. For more, see flutter#139613, which tracks this inconsistency in behavior. 

This means that in the scenario where

1) the user does not have Android Studio installed or the java binary found within cannot be run **and**
2) the user has a) `flutter config --jdk-dir` set, b) `JAVA_HOME` set in their environment, **or** c) `java` on their system path **and**
3) the java binary we think we found during cannot be run (or `java --version` fails), **then**

the user running `flutter create` with Android enabled will hit a tool crash.

## Change

`Java.version` should return null if version checking fails for any reason. [This is documented behavior](https://github.com/flutter/flutter/blob/48f57621ade657b0c20ba53d513de4c3cd563abd/packages/flutter_tools/lib/src/android/java.dart#L136). Therefore, we'll update the implementation to first verify that the binary is runnable. If it isn't, it will return `null`.

[^1]: We find java by calling the static `Java.find`, see: https://github.com/flutter/flutter/blob/48187028c11ca8ca10e0179705d25553e1fe2c14/packages/flutter_tools/lib/src/context_runner.dart#L271
[^2]: This PR doesn't change this, as this would be too dangerous to cherry-pick into stable.