Remove gwpc_aware_windowing_mode flag.

The `gwpc_aware_windowing_mode` flag is removed, making the behavior previously enabled by the flag the default. This means `DisplayWindowPolicyController` no longer includes `WINDOWING_MODE_MULTI_WINDOW` in its default supported windowing modes, and `GenericWindowPolicyController` will not block activity launches for unsupported windowing modes, allowing a fallback to fullscreen.

Fix: 434679423
Test: presubmit
Flag: EXEMPT cleanup
Change-Id: I1a3467da8e4e4a751352b995260840d90d6c7407