Notification: Reuse drawable in Header if Icon unchanged

Mitigates an issue where a LevelListDrawable would constantly
be reloaded even if unchanged. To avoid this, small icons are
now only reloaded if they no longer point to the same resource.

Note that StatusBarIconView already has this logic.

Change-Id: I6be436e5cef7b7ca91a28edc413b1aaa0f1007d5
Fixes: 30496073