Refactoring in SafetyCenterDataRepository

These are non-functional changes in preparation for the bug below
which adds new logging to the classes modified here. In working out
how to do that it seemed to me that some of the existing documentation
had become kind of innaccurate and I wanted to fix it.

Bug: 268309211
Test: atest SafetyCenterManagerTest
Change-Id: I6d382918b615b3442064499807e08890f7ebd1bd