service_contexts: label service_contexts explicitly

The label applies to all service_contexts regardless of their location.
This also lets us track the service_contexts usage and limit access to
the files for the corresponding object manager alone.

Bug: 36002427
Test: Boot sailfish and observe no denials for 'serice_contexts'
Test: cts-tradefed run singleCommand cts --skip-device-info \
          --skip-preconditions --skip-connectivity-check \
          --abi arm64-v8a --module CtsSecurityHostTestCases \
          -t android.security.cts.SELinuxHostTest#testAospServiceContexts

Change-Id: I97fc8b24bc99ca5c00d010fb522cd39a35572858
Signed-off-by: Sandeep Patil <sspatil@google.com>