stagefright: Fix issue with tracking media format in packet source

Media format in AnotherPacketSource is now tracked across discontinuities.
This fixes a bug where format was set on queueAccessUnit and cleared on
dequeueAccessUnit, thereby allowing it to remain cleared.

Change-Id: I20975a630443f4a223a2b4344e8244f34b9560b9
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 12060952