Re-land Android fullscreen support

[Piinks]

This re-lands flutter/flutter#25785. The original PR was breaking tests internally due to a change in SDK support for navigation bar color.

While refactoring the SystemChromeStyle, I noticed that changing the navigation bar color was available at an earlier sdk than we were allowing, so I updated it to match native expectations. (changed from 26 to 21 where earliest available).

This is what broke tests internally on older devices. While it ends up giving the correct behavior for these older versions of Android, it made me realize why we were limiting this in the first place. The customer test has a perfect example:

SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark.copyWith(
        systemNavigationBarColor: Colors.white,
        systemNavigationBarIconBrightness: Brightness.dark,
        statusBarIconBrightness: Brightness.dark));

The ability to change the navigation bar icon colors is only available at sdk 26. So if we were to allow the navigation bar color to be set, you could have white buttons on a white background like in this test. This is not desirable and is probably why it was this way to begin with. I have reverted the change to the way it was previously and documented this.

This is the diff from the original change: https://github.com/flutter/engine/pull/27018/files/ca8e05f563daa8707b25047b48a3c1ddf7ac9242..0f31f8bf4debcdf977d030871dca41ffe0792ee4

---

From Original PR (#25785)

This adds support for Android fullscreen modes.

Framework side: flutter/flutter#81303

Related Issues:

• Support for translucent + transparent Android's SystemNavigationBar flutter#35748
• Gallery should implement full screen / edge-to-edge for Android Q+ devices gallery#643
• [Android 10] Unable to set transparent navigation bar flutter#40974
• Bottom gap on Pixel 4 flutter#44033
• Android 10 incorrect insets when navigation bar is translucent flutter#63761
• Flutter still doesn't support modern Android Q-style render-behind-navbar styling flutter#69999

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[Piinks]

Running through internal testing to make sure this is resolved now. :)

[blasten]

LGTM

[gaaclarke]

Requested changes are just the aforementioned comments.

[gaaclarke]

lgtm

[flutter-dashboard]

This pull request was opened against a branch other than main. Since Flutter pull requests should not normally be opened against branches other than main, I have changed the base to main. If this was intended, you may modify the base back to master. See the Release Process for information about how other branches get updated.

Reviewers: Use caution before merging pull requests to branches other than main, unless this is an intentional hotfix/cherrypick.