[flexiglass] Makes scene_container an aconfig flag.

Converts the existig classic flag scene_container into an aconfig flag.
Eliminates the SCENE_CONTAINER_ENABLED constant from Flags.kt as that's
no longer needed to enable Proguard to remove Flexiglass at compile
time, now that we have a compile-time static function that checks for
the flag.

Bug: 283121968
Test: used the updated flexiglass.sh script from cl/574663066 to test
turning flexiglass on and off

Change-Id: I309c30b7f23082c1566101d539620f5c92d69f14