Always parse sharedUserId and sharedUserLabel

In preparation to guard the "leaving shared UID" feature behind a flag,
we should always parse shared UID related attributes. With these
changes, we can disable the ability to leave shared UID simply by
ignoring the "sharedUserMaxSdkVersion" attribute during parsing.

Test: atest SharedUserMigrationTest
Bug: 220015249
Change-Id: Ia355f334311533158c706dd052d6f7b4eb611cca