Make Condition use CLOCK_MONOTONIC

Changes to the REALTIME clock can cause Condition::waitRelative() to
timeout early or delayed.  This behavior is undesired and new since
https://android-review.googlesource.com/#/c/181058/ moved the
underlying pthread_cond_timedwait() implementation to use absolute
timeouts rather than relative ones.  Having Condition use
CLOCK_MONOTONIC prevents these timeout issues.

Bug: 34592766
Bug: 35678943
Test: Boot bullhead
Test: Ensure time changes do not cause Condition::waitRelative() to
timeout early or delayed

Change-Id: I3a8d7a48f9b42fe990c3c7331313b6d85aa546f9