WidgetPicker - enable enterprise string cache in standalone activity.

The WidgetPickerActivity wasn't exposing a string cache for enterprise
overrideable strings, meaning that default fallback messages were showing
when the picker was in standalone (not in the launcher) mode.

Bug: 358002534
Test: Manual
Flag: EXEMPT bug fix

Change-Id: I67ccd46370708fb70a3a5233929a9c3b33c312f7