Android: Expose VideoCapturer.isCapturing() and add tests

Add a boolean isCapturing() method to the VideoCapturer interface and
implement it in FileVideoCapturer, ScreenCapturerAndroid, and
CameraCapturer. Allows callers to query capture state without tracking
it externally.

Tests:
- FileVideoCapturerTest: lifecycle assertion (start -> stop).
- CameraVideoCapturerTestFixtures: shared start/stop/restart helper,
wired into Camera1 (byte buffer + texture) and Camera2 tests.

Bug: webrtc:512514764
Change-Id: I576fec9721b69e328fdd8d2cd91c819cf67c19f4
Co-authored-by: Danilo Bargen <danilo.bargen@threema.ch>
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/472200
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48054}