Moved zen-conversion-related methods into ZenAdapters

This includes any "pure" Policy<->ZenPolicy conversion function that doesn't require internal state of the callers.

(Also moved the class so it's accessible to Settings).

Bug: 309922141
Test: atest ZenModeHelperTest ZenAdaptersTest
Change-Id: Ieadd237bb7d7ab5ea41c76f2770edc1af68a0510
8 files changed