[bc25] Add a helper for reading and using the Dual Shade aconfig flag.

The first usage will be included in an upcoming follow-up CL.

Test: It builds.
Bug: 337021582.
Flag: ACONFIG com.android.systemui.dual_shade DISABLED
Change-Id: Ibe9c091572882cfe5cf70c58ac12b77128b37727
1 file changed