Add support for showing a "None" item for non-exclusive roles.

This change allows adding a `showNone="true"` attribute to an
exclusive role, so that when the default app UI is shown, there is a
"None" item for user to clear the default app. Also fixed a bug in
getQualifyingPackagesAsUser() which results in NoSuchElementException.

Bug: 110557011
Test: build
Change-Id: I7ed267645cf90d50f63c044971754123f345662a