Add a new hidden Icon API for loading

The new API checks (for URI based Icons) that the Uri can be accessed by
the passed callingUid, regardless of the app that is calling
loadDrawable. This allows SystemUI to load drawables on behalf of other
apps safely.

Fixes: 301110522
Test: atest IconTest
Change-Id: Iae392277aaf9560536a0b1cf231fa98b577a32d7