Handle upgrade and B&R compatibilty

If loading xml that's an earlier version (either from B&R
or an OS upgrade), then migrate the app importance setting to
permissionmanager.

When writing XML for B&R, include app level permission values from
permissionmanager rather than local storage. Although permissions
are B&Red separately this information needs to be included in the
notifications backup in case it's restored onto an earlier OS.

Test: PermissionHelperTest/PreferencesHelperTest
Bug: 194833441
Change-Id: Id1271c538f524b90ee8c9b4eb447f872958b322c