Hide ZenMode.getRule()

This makes the ZenMode interface smaller, which would allow us to 1) have a ZenMode that does not correspond to an AZR, or 2) having an "incomplete" ZenMode with only the bits that SystemUI would need.

Also added some missing test coverage as part of the reshuffle.

Bug: 393321037
Test: atest SettingsLibRoboTests
Flag: EXEMPT Unflaggable refactoring
Change-Id: I9d07c52d5c04cc3e58eb9a1f7110d03e9c298100