Hide Flexiglass Compose code behind a constant Boolean flag

This CL adds an additional SCENE_CONTAINER_ENABLED flag on top of the
existing SCENE_CONTAINER flag that has to be flipped in source code when
developping to enable Flexiglass. This is necessary so that we can
enable our existing Compose features without paying the apk size cost of
Flexiglass (see comments of http://ag/24151185 for details).

Bug: 238993727
Test: Manual, checked the size of our userdebug APK with and without the
flag.
Change-Id: If9788a535a6f56b7f3aa44563d2c86e7c20bc687