GraphicsTracker: do not call cancelBuffer() after being stopped

When two codecs are using the same surface as below scenario, a buffer in Codec #2 can go inconsistent. Eventually the process using Codec #2 will be aborted. Fix this by not calling cancelBuffer() after GraphicsTracker is stopped.

1. Codec #1: stop using the surface (disconnect)
2. Codec #2: start to use the surface. (connect)
3. Codec #1: cancelBuffer() to the surface in HAL block dtor
4. Codec #2: a buffer by dequeueBuffer() is gone mistakenly by 3

Test: run cts -m CtsMediaStressTestCases

Bug: 354074267
Flag: EXEMPT bugfix
Change-Id: I8489a26021f456a5e9909a3546fc09c958053608