Add the mutability flag for SettingsSliceProvider

Starting Android S, all PendingIntent should have the mutability flag
assigned to prevent from vulnerability.

Fixes: 172811605
Fixes: 172206649
Test: 1. Play on the slices in Settings search.
2. Make a permission request intent of Settings wifi slice in the
slice-viewer app, clicking on it and make sure it gets redirect to
Settings without crashes.

Change-Id: I86f915bc062a6e632b5ca9c74e232db1036f08de