Don't disable "Done" button when it cannot be pressed

Show an error on tap instead. According to a11y guidelines this is preferrable.

Fixes: 369942776
Test: atest ZenModeEditDonePreferenceControllerTest
Flag: android.app.modes_ui
Change-Id: Ib5e47a4151c1515e5085a776d538a27c2ef41574
2 files changed