seapp_context: explicitly label all seapp context files

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

Bug: 36002414
Test: no new 'seapp_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#testAospSeappContexts
Test: Launch 'chrome' and succesfully load a website.
Test: Launch Camera and take a picture.
Test: Launch Camera and record a video, succesfully playback recorded
      video

Change-Id: I19b3e50c6a7c292713d3e56ef0448acf6e4270f7
Signed-off-by: Sandeep Patil <sspatil@google.com>