Make AndroidColorScheme easily overridable

This CL is a small refactor of the AndroidColorScheme so that it can be
easily overridden using CompositionLocalProvider. This will be used by
Magic Portrait to locally change the colors of the app when previewing
new shape colors.

Bug: 355452966
Test: atest PlatformComposeCoreTests
Flag: NONE trivial refactoring
Change-Id: I4d908f1c76b929f865c41e8ac8e7541b26921626