Customize the text reading preview layouts

Make it possible to customize the available text reading preview
layouts through overlays.

Bug: 307481249
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
"com.android.settings.accessibility.TextReadingPreviewPreferenceTest"
(cherry picked from https://android-review.googlesource.com/q/commit:2a7da9c433f57db9b7b9ad5c54bddb5da81e5693)
Change-Id: I3e55eeb7545a4d5d1f689ab4160e453996b58bdb