Bind-mount individual flattened APEXes

There are a few directories for builtin APEXes: /system/apex,
/system_ext/apex, /product/apex, /vendor/apex.

For devices with ro.apex.updatable=false, init should bind-mount
"flattened" apexes from all of above.

Bug: 144732372
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m && device boot
Change-Id: If151145e6e3d966b202300ff3a0ddb5ec9fdaa87