Include documentation for a null surface in TvInputService.Session.onSetSurface.
Include documentation for a null surface in TvInputService.Session.onSetSurface.

Bug: 18718144
Change-Id: I359a918da4ceeb01e44b38e6e82a501c0360e681

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base