Use file_contexts for flattened apexes

Because flattened apexes are installed in /<partition>/apex, associated
file_contexts file should be modified to accommodate path changes and
also merged into target-wide file_contexts.bin file.

Soong now emits modified file_contexts via LOCAL_FILE_CONTEXTS, and
these files are collected into a temporary file_contexts.modules.tmp
file, which is used as an additional input of file_contexts.bin.

Bug: 166518492
Test: m file_contexts.bin
      check the contents of file_contexts.concat.tmp file
Change-Id: I002a2294af9e06baeaf54bc5a9a5380d23b8a3da