Add sysui feature flags for two dream features.

1. A flag to enable hosting dreams within sysui (e.g. the lockscreen)
   rather than using DreamActivity.
2. A flag to enable using a DreamController hosted in sysui rather than
   the DreamController in system server.

Test: manually (not really anything to test until the flags are
enabled).
Bug: 288287730, 286092087

Change-Id: I98af897905ba1f8e6108d5373a6bee73d97b5a50