Separate diffing functionality into a separate class.

This creates ZenModeDiff with subclasses for diffs on ZenModeConfig and ZenRule. The new class allows for both emitting a string describing the diffs as well as reading what the diffs were after the fact (which the previous implementation did not allow).

Bug: 259261349
Test: ZenModeDiffTest, adb shell dumpsys notification

Change-Id: I8d51a8dc6c0edf548e1ef8ba350d9302d9ca7a86