Wait until the thread is waiting before issuing next task

The previous logic doesn't guarantee the persister queue treating the
second task as a new batch of tasks. Hence it isn't guaranteed to wait
pre-task delay before performing the second task.

Bug: 387899271
Test: atest --iterations 10 WmTests:PersisterQueueTests
Flag: EXEMPT test fix
Change-Id: Ia5a423a4f2c54d1e4931746d99e9fbb0974fe638