Improve concurrency access for updateMetrics()

nuplayer's updateMetrics() referenced an unprotected shared stats buffer.
It's a small buffer, so we now make a copy during updateMetrics()
[at a point where we are mutexed] instead of putting a mutex on the
underlying frequently used construct.

Ensure that nuplayer2 has the same protections.

Bug: 123256408
Test: race condition
4 files changed