Modify accessibility label of ThemeableSelectionView based on its selection state

[adincebic]

When user was creating a new account, plan selection was not indicated to VoiceOver users. When user taps "regular" or "plus" account, VoiceOver now prefixes with word "selected" if that plan is selected. This is inspired by UIButton selection state.

To test

1. Turn voiceover on
2. Go to profile
3. Tap create account
4. Navigate with voiceover

If selected plan is regular, voiceover should say "Selected, Regular". If regular is not selected, voiceover should just read the accessibility label "regular". Same of course applies for the plus plan.

[CLAassistant]

All committers have signed the CLA.