[2/n] Introduce main abstraction behind flag

Introduces main compat ui framework abstraction adapting existing code
behind a feature flag.

Flag: com.android.window.flags.app_compat_ui_framework
Fix: 304235607
Bug: 270361630
Test: atest WMShellUnitTests:ShellTaskOrganizerTests
Test: atest WMShellUnitTests:CompatUIControllerTest
Test: atest WMShellUnitTests:CompatUILayoutTest
Test: atest WMShellUnitTests:CompatUIWindowManagerTest
Test: atest WMShellUnitTests:LetterboxEduDialogLayoutTest
Test: atest WMShellUnitTests:LetterboxEduWindowManagerTest
Test: atest WMShellUnitTests:ReachabilityEduLayoutTest
Test: atest WMShellUnitTests:ReachabilityEduWindowManagerTest
Test: atest WMShellUnitTests:RestartDialogLayoutTest
Test: atest WMShellUnitTests:RestartDialogWindowManagerTest

Change-Id: Ib3156947b7d0761babef03a28fb4c5ecf64ba191