Add libselinux keystore_key backend.

We add a new back end for SELinux based keystore2_key namespaces.
This patch adds the rump policy and build system infrastructure
for installing keystore2_key context files on the target devices.

Bug: 158500146
Bug: 159466840
Test: None
Change-Id: I423c9e68ad259926e4a315d052dfda97fa502106
Merged-In: I423c9e68ad259926e4a315d052dfda97fa502106
11 files changed