C2BqBuffer: Remove detach during migration

Remove detach for efficient surface switching.
1. Since GraphicBuffer handle is cloned, removing detach() from
   migration is fine.
2. Since we will not use dequeued GraphicBuffer after BlockPool dtor(),
   removing detatch() from dtor is also fine.

Bug: 185451746
Test: atest android.media.decoder.cts.AdaptivePlaybackTest
Test: manually crafted test
Merged-In: I46cf14e7e71e35a5982d8073707bbbf558be9be6
Change-Id: I46cf14e7e71e35a5982d8073707bbbf558be9be6