libmediametrics: fix a memory leak

The memory returned here is malloc()ated, and it's not clear that
submitBuffer is intended to free it. Do so.

Caught by the static analyzer:
> frameworks/av/media/libmediametrics/MediaMetricsItem.cpp:287:16:
warning: Potential leak of memory pointed to by 'str'
[clang-analyzer-unix.Malloc]

Bug: 179814879
Test: TreeHugger tests
Change-Id: Id0320fb7fe4567b79131884f73e2a73b153039d8