Add system_ext/apex/*/bin to fs_config

Flattened APEXes can be installed in /system_ext.
Especially, GSI have both flattened/unflattened apexes so that it can
run with updatable devices and non-updatable devices as well.

Bug: 145963716
Test: flash GSI on P device and boot
Change-Id: I3a941672446a5353517243a851739a66f4252617