file_context: explicitly label all file context files

file_context files need to be explicitly labeled as they are now split
across system and vendor and won't have the generic world readable
'system_file' label.

Bug: 36002414
Test: no new 'file_context' denials at boot complete on sailfish
Test: successfully booted into recovery without denials and sideloaded
OTA update.
Test: ./cts-tradefed run singleCommand cts --skip-device-info \
       --skip-preconditions --skip-connectivity-check --abi \
       arm64-v8a --module CtsSecurityHostTestCases -t \
       android.security.cts.SELinuxHostTest#testAospFileContexts

Change-Id: I603157e9fa7d1de3679d41e343de397631666273
Signed-off-by: Sandeep Patil <sspatil@google.com>