Clean up some edge cases in the compat mode UI:

- Don't put up multiple dialogs if a switch happens between activities
  that both can use compat mode.
- Remove the help dialog if the user switches away from an app that
  can use compat mode.
- Don't set the "don't show" preference until the user explicitly hits
  the button.
- Hide the compat mode switch UI if the compat mode button is hidden.

Change-Id: I8dd2aab1385a264232a9256284d4a75b8e058e47