Extract SurfaceCapture from ScreenEncoder

Extract an interface SurfaceCapture from ScreenEncoder, representing a
video source which can be rendered to a Surface for encoding.

Split ScreenEncoder into:
 - ScreenCapture, implementing SurfaceCapture to capture the device
   screen,
 - SurfaceEncoder, to encode any SurfaceCapture.

This separation prepares the introduction of another SurfaceCapture
implementation to capture the camera instead of the device screen.

PR #4213 <https://github.com/Genymobile/scrcpy/pull/4213>

Co-authored-by: Romain Vimont <rom@rom1v.com>
Signed-off-by: Romain Vimont <rom@rom1v.com>