Camera: Fix deadlock in ACameraCaptureSession destructor

mSequenceCallbackMap's entry should be cleared before calling
onSequenceCompletedCallback. This is to make sure the
ACameraCaptureSession object is destroyed in the callback thread.

The fix is for a regression introduced in ag/11835321.

Test: Camera CTS
Bug: 176034707
Change-Id: Ib38f4ca8ec499cd88f31b0be09f1db74e5ec35c6