apex: emit file_contexts for flattened apex

Flattened APEXes are installed in /<partition>/apex/<name>, so its
file_contexts file should be amended accordingly.
For example, file_contexts of an apex "com.android.foo" with should be
modified by prepending actual install path to the path regex.

/bin/foo <sepolicy label>
=> /system/apex/com\.android\.foo/bin/foo <sepolicy label>

File_contexts file is emitted to "make" via LOCAL_FILE_CONTEXTS variable
for apex_manifest.pb module.

Make collects these files and use them to build file_contexts.bin.

Also removed global state which is used to keep the list of all
flattened apexes.

Bug: 166518492
Test: m file_contexts.bin
      check intermediate file_contexts.modules.tmp file
      in $OUT/obj/ETC/file_contexts.bin_intermediates/
Change-Id: I780370b6e576964a5c09f9477f3361f341dd576b