Marks Settings.mPackages as VisibleForTesting

mPackages was accessed directly throughout the pm package. This change
removes those from non-test code and marks the field @VisibleForTesting
to account for the tests still accessing it directly.

Bug: 169360984
Test: atest PackageManagerServiceBootTest
Change-Id: I85be9daabd01f49f86b2cc0d5f9e2bc3d090f52b