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.
580eb4fe38 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
2dfd54d07e