audio: Simplify AudioHalBinderServiceUtil::waitForFired()

Return the std::condition::wait_for() return value instead of reading
the variable 'fired' again. See also
https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for

.

Change-Id: I48c270f5446f61b2ad828b19a1fc294737a123dd
Signed-off-by: Bart Van Assche <bvanassche@google.com>