Add locking to AVRCP Target JNI

This prevents a race condition from occuring where a call can be sent
form native while or after cleanup has occured.

Bug: 78517259
Test: Compile and switch users with multiple devices connected
Change-Id: Ibde219746bb1ade2fc90c366bf63b0c0f1ea27af
(cherry picked from commit cbabc47e)