Store rule icon res name

Test: NotificationManagerZenTest
Test: ZenModeHelperTest
Test: ZenModeDiffTest
Test: test app that creates rules with icons and shows them,
used directly and to confirm icon state after reboot
Test: make system rules add icons, confirm display in (patched) Settings
Fixes: 308672670

Change-Id: Ib55cda476a1a1e97433694cab3d6f9b4a95f86e2