Add permission to load library from system_ext

Bluetooth a2dp vendor codec are manage by the presence (or absence) of
the corresponding `.so`. Since we moved as an apex, the `.so` cannot be
found in the current directory.
This CL try to load each `.so` from various location.
This CL add the `/system_ext/${LIB}` to the list of permitted_paths, to
allow dlopen of aptx & aptxHD library.
We ensure we don't load a 32 lib in a 64 module thanks to the
permitted_paths.

Test: build + start bt and watch log
Fix: 223532240
Tag: #refactor
Ignore-AOSP-First: No apex on aosp
Change-Id: Ie1dd9877edc905230a4f6d05b991e26c9908f8c7