Install external deps of an APEX

By default, if a lib is included in an APEX, all its direct and indirect
dependencies are also included in the same APEX. However, when one of
the dependencies have stable API (i.e. has stubs: {...}) then the lib
having stable API and its dependencies are not included in the APEX.

However, the problem here is that the lib having stable API might not be
installed on the system, thus causing error at runtime. This can happen
if there is no other module in the platform that depends on the lib.

This change fixes the problem by adding such libraries as external
dependencies so that they are also installed on the device along with
the APEXes using them.

Bug: 124831003
Test: m installclean; m com.android.resolv
libbinder_ndk, libvndksupport are found under system/lib

Change-Id: I457e03ff3fce37e0890c64d911e6e0ea6d0c6dd6