Prevent proguard from pruning ClockSetting

ClockSetting is populated via gson (using reflection), so proguard
will clean it. This causes gson to silently fail and return and
empty object in some configurations. Annotating the class with
@Keep prevents this from occuring.

Bug: 229771520
Test: Manually tested on userdebug variant
Change-Id: I9a7c3e4c4d66d1c13d58cba5994d5578034a9bf8