Fix UnsupportedErrorDialogTests minor mistakes

* testDevSettingPrecedence needs to use the right API to set secure
  settings (current code looks like a victim of last-minute changes
  before submitting the patch)

* testAnrIsNotShown should wait a bit longer for ANR. This takes between
  5 and 10 seconds. We poll for at least 7 seconds to avoid flakiness
  and also send TAB instead of BACK to avoid risk of dismissing a
  dialog.

    * Expecting ANR to happen immediately after BACK finishes was
      reasonable when key events could be blocked but that is not the
      case any more.

Fix: 169155905
Change-Id: Icf0d86974315b069259192984556c53e7040cb0e