JNI: Fix crash in btgattc_scan_result_cb

This crash happens when we receive scan result while shutting down BT
stack.

We should be obtaining mutex before calling any callbacks to prevent
such crashes.

Test: scan in noisy environment, stop BT stack multiple times
Bug: 246518104
Change-Id: I7fe7e0f7b9e69b24cf8b41a99dabfb00119080f3