New feature “Text and reading options” for SetupWizard, Wallpaper, and Settings (4/n).

- Extending the interface of AppGridView to be flexible. It will be used
in the TextReadingPreferenceFragment.
  1. Add a new attribute "appCounts" into the view.
  2. Default app counts as 6 if not set the attribute.

Bug: 211503117
Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER=AppGridViewTest
Change-Id: I94a5f1d037551f6faba0300a176feb11fba1f203