Make createSource(ContentResolver, Uri, Resources) a SystemApi

Bug: 152322291
Test: make

Once ImageDecoder is in a mainline module, the system (i.e. ImageView)
needs to be able to access this method.

External clients can already use createSource(ContentResolver, Uri). If
they want density scaling, they can use
createSource(Resources, int resId).

Change-Id: I560972c1ef08ee5e610e3c2c182353a6767fd082