use dedicated looper for GenericSource

- handle setVideoSurface in deferred action, and return
  to client immediately

- handle GenericSource's prepareAsync on its own looper,
  instead of sharing looper with NuPlayer

- let HTTPLiveSource share looper with LiveSession, instead
  of NuPlayer

- remove reflector in RTSPSource

Bug: 16892748
Change-Id: I1aed557320052012065f5a90adbcb03c238da988