Start partitioning off privapp_data_file from app_data_file

Currently, both untrusted apps and priv-apps use the SELinux file label
"app_data_file" for files in their /data/data directory. This is
problematic, as we really want different rules for such files. For
example, we may want to allow untrusted apps to load executable code
from priv-app directories, but disallow untrusted apps from loading
executable code from their own home directories.

This change adds a new file type "privapp_data_file". For compatibility,
we adjust the policy to support access privapp_data_files almost
everywhere we were previously granting access to app_data_files
(adbd and run-as being exceptions). Additional future tightening is
possible here by removing some of these newly added rules.

This label will start getting used in a followup change to
system/sepolicy/private/seapp_contexts, similar to:

  -user=_app isPrivApp=true domain=priv_app type=app_data_file levelFrom=user
  +user=_app isPrivApp=true domain=priv_app type=privapp_data_file levelFrom=user

For now, this newly introduced label has no usage, so this change
is essentially a no-op.

Test: Factory reset and boot - no problems on fresh install.
Test: Upgrade to new version and test. No compatibility problems on
      filesystem upgrade.

Change-Id: I9618b7d91d1c2bcb5837cdabc949f0cf741a2837