Suppress failing Settings Robolectric tests

- Assume API 24 for some settings robolectric tests since they are
broken by referencing API new in version 24. Those tests will be
ignored until Robolectric is updated to support SDK 24.

- Comment out failing DatabaseIndexingManagerTest.

Test: cd tests/robotests && mma
Change-Id: Id71211b7a6836d55f31524c4cfe9628c2de78097