Flush buffered data when session connects.

   Typically this data would have been flushed at the end of the
   first draw and there would otherwise be an extended delay
   before the data would be sent to the service.

Test: make -j
Test: Manually start WhatsApp - check for lost events
Test: atest CtsContentCaptureServiceTestCases

Bug: 154777879
Change-Id: Ia0a97e24ffcc1c189d5d0cd6b8888678fa48968e