Change flag type to control Quickstep widget app launch

Previously we'd relied on a SystemProperties to control which
InteractionHandler was used by Quickstep for widget app launches.

This switches to a more conventional FeatureFlag.

Bug: 169042867
Test: manual
Change-Id: I7abf815c6e204daf996ac0a81b1a354c4e3d5cda
2 files changed