The latest Android 10 activity transition occurs over 400ms. Currently, MaterialPageRoute has a transition duration that is set to a constant of 300ms.

We should create a new parameter so that developers can modify/customize the transition duration if they'd like. This also allows Flutter's Android 10 transition to have the correct transition duration.

• Add MaterialPageRoute.transitionDuration parameter
• If possible, add a new PageTransitionsTheme.transitionDuration property that is correctly looked up by all page route classes so that it needs to only be specified in one place instead of wherever a new page route is defined.