audiohal: Check for EINTR and retry when waiting for EventFlag

EINTR can be returned in the case when a futex wait has been
interrupted by a signal. We need to retry in that case.

Bug: 35813113
Change-Id: I7eaee5298f5c0bc5cd62309c3fd8349ee3eae629
Test: make
(cherry picked from commit d2ae9cd5)