libstagefright: encoder must exist when source starting.

when trying to start source, need to make sure encoder still exist.
Although encoder starts before source, it may encounter error and be
released before starting source, this will cause camera hardware can't
be released.

Author: Cong Jiajia <jcong@codeaurora.org>

Bug: 70919418
Test: TBD
Change-Id: I68b0d3a86d7004d4d1b3aefb580093a6497dc485