Never disable the manual rule (and fix it on read if it was disabled)

ZenModeConditions could potentially do this if you flipped modes_ui from true to false.

This would manifest as the DND mode refusing to activate (from any surface) when modes_ui was flipped to true again.

Fixes: 366451998
Test: atest ZenModeConfigTest
Flag: EXEMPT Bugfix for modes_ui
Change-Id: I63f51568ae4d556c4536d5620fc70988af8a9570