Fix a memory leak in the osi allocation tracker

Add a missing free(allocation) inside allocation_tracker_notify_free()
when freeing a memory buffer.

Prior to switching the allocation tracker to use C++ unordered_map,
the "allocation" bin was kept within (and reused) by the older hash_map
mechanism. However, after the switch to the C++ unordered_map this
is no longer the case, hence the memory leak.

Test: watch -n 1 "adb shell dumpsys meminfo -d BT-PID | grep Heap"
Bug: 34785845
Change-Id: I737c901a6452d29d18fd8b847e7d5ea10c5ff485