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Split the directories out per-user. Each Android user ID gets its own
directory and master key. This gives each user its own locked/unlocked
state.

Add migration code that converts existing keystores to this scheme. This
even migrates keys that used the non-public API, but only for the
primary user. The secondary users may have a different lock screen
pattern that would no longer work to unlock the master key.

(cherry picked from commit 655b958e)

Bug: 7249554
Change-Id: Icfc117d9bc68bd88997ab03673fb77b581acf3f7