Fix tests for platform variant

RichTextView was only available in compat variants because of
ExploreByTouchHelper. This creates a copy of RichTextView for
platform builds without the accessibility features.

Also make RichTextView extend AppCompatTextView so it will inherit
support lib customizations.

Test: ./gradlew connectedAndroidTest
Change-Id: Ifce63a42aeb9bc25c543f4a5b9abea72435a76d6