Camera: fix signalStreamFlush logic

1. Fix off by one error in signalStreamFlush call
2. Make sure signalStreamFlush is called before we toggle request
   thread idle (which might cause another thread finishing
   waitUntilIdle() and thus start calling configureStreams)

Test: Pixel 3 + camera CTS
Bug: 120986771
Change-Id: Ifd6f77ef628ee200c024c7c6a05bde20937c745d