Camera: Add flush test cases

Camera should flush pending requets and buffers
correctly. Two new test cases will help with
 the verification of this method:
- 'flushPreviewRequest' - Constructs a valid camera request
and sends it to camera. The test will not wait for the request
to complete instead it will call flush immediately. The results
 and notifications that follow are checked for consistency with
expected behavior.
- 'flushEmpty' - Triggers flush without any pending request. Check
 whether the camera behaves as expected.

BUG: 32022758
Test: compile and run the gtest binary on device

Change-Id: Iab9e37b3864491f970a783a68952a9c0c1703520