Ensure long wait time for the first item in PersisterQueue

Unit tests revealed a race condition where mNextWriteTime set in the
first PersisterQueue#addItem() can be overwritten by
PersisterQueue#processNextItem() if the item is added quickly after
starting the writer thread. This could defeat the overall purpose of
having a long wait to avoid competing with critical resources system
boots need.

Bug: 402243483
Test: atest --iterations 20 PersisterQueueTests
Flag: EXEMPT bug fix
Change-Id: I6ca44618999aca4674d6a8fbc5e02592dd25169a