[1/n] Add build and runtime flags for fullscreen option

Introduces config_appCompatUserAppAspectRatioFullscreenIsEnabled as a
build time flag and "enable_app_compat_user_aspect_ratio_fullscreen" as
a runtime flag to guard fullscreen option in user aspect ratio settings.
Can only be enabled if user app aspect ratio settings flag is enabled.

adb shell wm set-letterbox-style --isUserAppAspectRatioFullscreenEnabled
[true|1|false|0]

Bug: 291900454
Test: atest WmTests:LetterboxConfigurationTest
Change-Id: Id7e8aa6a82e79d3e5a8f8ab6855b4d574c01551c