Pull keychain-data policy out of system-data

Migrators should be allowed to write to /data/misc/keychain in order
to remove it. Similarly /data/misc/user should be writable by system
apps.

TODO: Revoke zygote's rights to read from /data/misc/keychain on
behalf of some preloaded security classes.

Bug: 17811821
Change-Id: I9e9c6883cff1dca3755732225404909c16a0e547