[1/n] Add build and runtime flags to guard user aspect ratio settings

Introduces config_appCompatUserAppAspectRatioSettingsIsEnabled as a
build time flag and "enable_app_compat_user_aspect_ratio_settings"
as runtime flag to guard user aspect ratio settings.

Fix: 287448178
Test: atest WmTests:LetterboxConfigurationTest

Change-Id: Ic742a325891c7bf07ddb95a633b2db38dfa5f241