Move staging value application logic to persistent_properties and add

unit tests

1, Previous implementation has the staged prop application done in
property_service, which caused a number of unnecessary changes which
including exposing apis like AddPersistentProperty. In addition, it made
the property_service logic complicated. A better design is to have the
staged value application done while reading the persistent properties
from file. This way, no change to property service. In addition, unit
test is much cleaner and efficient.

2, add a unit test to lock down the behavior. Specifically, it locks down that when a prop is staged, it should be applied the next time when the persistent prop is loaded. In addition, it should lock down that other persistent props are not overwritten.

Bug: b/307752841, b/300111812, b/306062513

Change-Id: I43c603efbb803195065dda3f0bc2145716302bbc