Fix double free of play policy in a race condition.

The mPlayPolicy can be freed twice if there is a race condition.
mPlayPolicy should be protected with a mutex lock.

SafetyNet logging is not added to avoid log spamming. The
mutex lock is called whenever a license request is made.
That can happen quite often.

Bug: 176168330

Test: sts-tradefed
  sts-tradefed run sts-engbuild-no-spl-lock -m StsHostTestCases -t android.security.sts.Poc21_01#testPocBug_176168330

Test: run sts test on master build
  run sts test from http://go/ag/13308312

Change-Id: Ibc338e0a98293807dbf12500f7e82e62b6c4a04a
(cherry picked from commit 537144f7)