Don't allow hiding SysUi

Hiding SystemUI makes the phone impossible to use even when done inside
managed profile.

Bug: 328182084
Test: manual with custom DPC
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:befa3c99b9571d8f28f4974e0c4adfe538eeba66)
Merged-In: I847cb6d69db4924b2b3ddb741ac61a8f065e7c78
Change-Id: I847cb6d69db4924b2b3ddb741ac61a8f065e7c78