apex: add apex_manifest.pb in file_contexts

apex_manifest.pb is added to payload.img by the build system and is
supposed to be read by apexd during activation.

So, it should be labeled as "system_file" so that apexd can read it
regardless of how file_contexts is defined.

Bug: 159767275
Test: add an apex with file_contexts with everything as vendor_file
      apexd should activate the apex.
Change-Id: Ie34eb7ad905828976152d67a7dfabf0add0a5de4