audioflinger: fix wake lock acquisition check

Commit e14a5d6d introduced a bug in ThreadBase::acquireWakeLock() where
the wake lock acquisition return code is not initialized before
being checked causing the wake lock token to be sometines not set
and potential failure to release the wake lock later.

Bug: 10985160.
Change-Id: Iffd40e773ae3bcfec3c148a844d5dbebbf474eaf