Ask password when user adds a new fingerprint including a checkbox to ask that

if they want to use fingerprints in the future and stores it as shared prefernces.

Including some refactoring.
- Keep the purchase button visible after a successful purchase so that users of
the app can test a flow after registering a new fingerprint.
- Fixed broken tests.
- Use end/start instead of left/right for RTL languages.

Bug: 21655960

Change-Id: I9b0015fa5f61048c611ab798afbf96edbc3239eb