Add feature flag + leftward swipe for Compose access.

Stole ag/9453040 from jspierce@ and added a feature flag.

ag/9453040: Exploratory prototype to test leftward swipe access to
Compose across home screen, launcher, and lock screen. Requires Compose
APK (installed separately).

Change-Id: I15a045976b1eb41392795d3a4f0743f365dec1d2