Unit test for CameraGestureHelper.

Also includes some massaging of `CameraGestureHelper` to be more
testable.

Test: Made sure that double-tapping the power button still brings up the
camera correctly in all the scenarios described in ag/19214502
Bug: b/235403546

Change-Id: I54e3f760b0c2fa5738766f8b97568caf07676a8d