Add flag to guard WM Extensions for all devices

1. All modules of WM Extensions are disabled for devices that don't
   support multi window.
2. Activity Embedding module is disabled for new Extensions devices
   unless the app's targetSDK is Android 15 or above.

Bug: 306666082
Test: atest WMJetpackUnitTests:WindowExtensionsTest
Test: atest FrameworksCoreTests:WindowManagerTests
Change-Id: Ia540f191392faea50ca4a68c673f4217a2e0fad7