Add PackageItemInfo Attribute extraction to PackageManager.

This changelist adds a (hidden) convenience method to PackageManager,
enabling extraction of attributes in the form of a TypeArray from
a service's meta data.

Test: ApplicationPackageManagerTest
Bug: 334083490
Flag: N/A
Change-Id: I7edfec1bca520fbe528f824dad86f2b2c3853c3f