Camera: implement signalPipelineDrain API

SignalPipelineDrain will be sent after the request
thread has stopped sending capture requests so
HAL can expect no more capture requests are sent
after they receive this call.

Test: CTS
Bug: 109829698
Change-Id: I6f75c28ff0998a8edc80f9af9ebe727c585ea6e9
2 files changed