Push mapping of package name to appId to kernel.

The new sdcardfs kernel driver needs to know this mapping for
deriving UID permissions, so push the data through /config when
supported by the kernel.  This also has the nice benefit of letting
us push only the deltas of what actually changes, instead of
re-parsing the entire "packages.list" file.

The mappings for newly installed apps are pushed before the app is
allowed to run, removing some latent race conditions.  Also cleans
up stale mappings when packages are uninstalled, and whenever the
system server reboots.

Bug: 19160983
Change-Id: Iace92efb69616c96b34c0d9d911e4b54e5fd8a67