[RESTRICT AUTOMERGE] C2AllocatorIon:protect mMappings using mutex

Use mutex to prevent multiple threads accessing same member of
mMappings list at the same time.

Bug: 193790350

Test: adb shell UBSAN_OPTIONS=print_stacktrace=1 /data/local/tmp/C2FuzzerMp3Dec -rss_limit_mb=2560 -timeout=90 -runs=100 /data/local/tmp/clusterfuzz-testcase-minimized-C2FuzzerMp3Dec-5713156165206016
Change-Id: I24e53629d5a6dfad22b84dd2278eb1a288c9ab35
(cherry picked from commit 9d2295f3)
(cherry picked from commit ac5f5cad)
Merged-In:I24e53629d5a6dfad22b84dd2278eb1a288c9ab35