Fix a bug in OperationScheduler#getNextTimeMillis()

moratoriumTimeMillis which is a wall clock time was wrongly
used as a limit for backoff, which was then added to another wall
clock time. This has resulted in getNextTimeMillis() returning the
time tens of years in the future.

Bug: 62161211
Test: runtest -c com.android.common.OperationSchedulerTest android-common-ex
Change-Id: Ie0ef7c9bda1194568e8c8bea820c3b6a601d209c