Add getPackageStates to replace forAllPackageStates

Rather than pre-optimize the filtering path, the FilteredSnapshot
now just returns a Map<String, PackageState>.

Any optimization can come in the form of a custom Map implementation
which caches on iteration, but that can be saved for when there's a
use case for that.

This leaves the old API intact for now due to a bulid system quirk
where the dependent CL can't be merged if the API in the prebuilt
doesn't exist.

Test: atest PackageManagerLocalSnapshotTest

For now a simple Map makes the API easier to use.

Bug: 246609797

Change-Id: Ifc68d4753290e2922672859b1a8a15b1ec0795cf