[flexi] Don't include Flexiglass if static flag is off.

This undoes the APK size bloat introduced by ag/24640909 where we
removed the use of Flags.SCENE_CONTAINER_ENABLED in
ShadeViewProviderModule which removed the ability of Proguard to cull
Flexiglass code when compiling.

Fix: 299199299
Test: presubmit shows BloatBuster is shows a smaller APK size.
Change-Id: Id481a7ebc4bf489fc56ba272797e7b0c57974f99