Fix bug where caller to bugreport mapping got lost

The mapping of calling app to deferred bug report was being lost in some cases which resulted in apps not being able to retrieve a bug report they previosuly requested. This was caused by the mapping being overriden on reboot by a new call to a deferred bugreport without the existing persisted mapping being read first.
This is now fixed by ensuring the mapping is read from disk first before updating it with new values.

Bug: b/391129113
Test: manual
Change-Id: Ib17f33361b962c5880508f9e52bf4f6132e32701