Fix DREAMING_STARTED broadcast.

This change fixes a bug where DREAMING_STARTED intent is only broadcast
once because mSentStartBroadcast flag is not being reset after
DREAM_STOPPED intent is broadcast.

Bug: 255732899
Fix: 255732899
Test: atest SystemDreamTest
Test: verified fingerprint unlock during dream works
Test: verified dream start/stop broadcasts are sent correctly
Change-Id: Ibcd8b8b00b855bfc52a295e2663926e55d664a69