Moved AttributionSource related APIs in AttributionSource

Modified myAttributionSource() to check for global AS for
process in ActivityThread and fallback to building new AS
with PackageManager#getPackageForUid(myUid()) if null.

Tag: #feature
Bug: 210467846
Bug: 210468546
Test: build
Change-Id: I7aa75395469bf0bb806100420faaf98c52057355
CTS-Coverage-Bug: 210906055