Insets cleanup

This adds some missing nullability and IntDef annotations, and fixes
some styling issues and IDE warnings. This also modifies class fields
to have hungarian notation, and changes some fields and classes to be
final, where applicable.

Additionally defines WindowInsets.Type.FIRST as being STATUS_BAR, rather
than the other way around, to adhere to the IntDef definition.

Flag: EXEMPT cleanup
Bug: 281029564
Test: presubmit
Change-Id: I8a563aa8fa6003135c44efbb19393c5e53203d40