Make FeatureFlagManager Dumpable

Also direct developers who find FeatureFlagManager in the src/ tree to the src-debug/ version

Fixes: 204127880
Bug: 203904245
Test: adb shell dumpsys activity service com.android.systemui
Test: FeatureFlagManagerTest
Merged-In: I6a42a697120179048e848f7f432fd753db514b9c
Change-Id: I5bec46e60befaa1890b9ff4d4bb5e89b0ebba256