Make libseliux a stub library

Make libseliux a stub library

libselinux is currently being copied to APEXes. This is risky because
the library is not designed to be portable; part of it is tied to the
specific version of the Android that it was developed for.

This change fixes the problem by declaring that the library supports
a stub with the list of C APIs that are included in the stub. Then there
is only one copy of libselinux in /system/lib and other APEXes use the
copy by dynamically linking to it.

Bug: 151053366
Test: m com.android.adbd. It doesn't include libselinux in it.
Test: m com.android.adbd-deps-info. then inspect
out/soong/com.android.adbd-deps-info.txt. The dependency to libselinux
is shown as '(external)'.

Change-Id: I445db2d0c5103b8b3267caa094f5d91259f1f491