[AChoreographer] Resolves a deadlock

This deadlock could occur when posting a delayed frame callback while a
pending refresh rate callback had not yet been processed by the looper
thread. Resolve the deadlock by releasing the lock in
Choreographer::scheduleCallbacks once the earliest dueTime is retrieved.

Bug: 150731776
Test: ChoreographerNativeTest
Change-Id: I9e1ace12beb4bbc75af0671ddf1b1653d04c9df4