apex: add / in file_contexts as system_file

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

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

580eb4fe adds /apex_manifest.pb and this
change adds / as well to file_contexts.

Bug: 159767275
Test: add an apex with file_contexts with everything as vendor_file
      apexd should activate the apex.
Change-Id: I827fa3af49ca7f20f3f82fcb16868f60d94d9531