Use weak pointers to prevent callbacks from executing after cleanup

Switch from using the instance directly to using weak pointers to the
instance. This causes any base::Callback objects that are bound to that
weak instance to be cleaned up if the weak pointer becomes invalid.

Bug: 78134184
Test: valgrind ./net_test_avrcp
--gtest_filter=*disconnectAfterCleanupTest* and see that there is an
invalid read before the fix and it is gone after.

Change-Id: I46c3175e62b87be165e468d014155f8d8154c08a