Force pthread_cond_timedwait_relative_np using CLOCK_MONOTONIC.

Previous patch changed pthread_cond_timedwait_relative_np to use
CLOCK_REALTIME, which causes app compatibility problem. So change
it back to CLOCK_MONOTONIC.

Bug: 30106240
Change-Id: I8e04058e92ede098f4f9f8d133f094001921441e
(cherry picked from commit fe4a4d8f)