MediaSession: Rename mCallbackHandler to mCallback

Support library uses reflection to the variable, mCallback, of
MediaSession. In order not to break the existing support lib
behavior on P devices, we need to keep the name.

Bug: 72979099
Test: version-compat-tests on P device
Change-Id: I5fd6c786d6a988777b94a3eb4f23d885202daa2a