Fix multiple instances for Google SignIn activity appearing in task stack

- Account page starts an activity with the flag FLAG_ACTIVITY_NEW_TASK
  to add an account and then causes multiple activity instances.
- Add the flag FLAG_ACTIVITY_CLEAR_TASK in conjunction with
  FLAG_ACTIVITY_NEW_TASK to clear existing task before the activity is
  started.

Fixes: 109923099
Test: robotest, manual
Change-Id: I8234ab33d8aa949e5ff78320962920cb0f8b2953