[flexiglass] Adds flag refactor wrappers.

This will make it easier to experiment with hard-coding flexiglass on
and off.

Bug: 283121968
Test: built and ran System UI, turned Flexiglass on and off via
go/flexiglass-script
Flag: N/A

Change-Id: I483841f594a4ad322497b5811d7a8fe19fddb8f5
54 files changed