MediaMetrics: Use libmemunreachable for memory testing

Full testing steps:

// required for unreachable
$ adb shell setenforce 0
// set up debug malloc
$ adb shell setprop libc.debug.malloc.program mediametrics
$ adb shell setprop libc.debug.malloc.options backtrace=8

// kill media metrics
$ adb shell pkill -f media.metrics
// verify media.metrics has restarted with new pid
$ adb shell pgrep -f media.metrics

// now check media metrics heap (largest 100 items) and unreachable memory.
$ adb shell dumpsys media.metrics --heap
$ adb shell dumpsys media.metrics --unreachable

Test: adb shell dumpsys media.metrics --heap
Test: adb shell dumpsys media.metrics --unreachable
Bug: 149850236
Change-Id: I269003485dd6933c399171fbd9d9a152cee50886