Always derive native library paths at runtime.

Over time, we've unpacked native libraries at various places with
respect to their source APK.  Persisting this path in PackageSettings
has caused more pain recently with the switch to supporting multiArch
and cluster installs.

This change switches us to always derive the native library paths at
runtime based on the type of install.  This also ensures that
transitioning between a bundled system app and an upgraded system
app will always build the right path.

We still persist the last generated path into PackageSettings to make
cleanup at uninstall time easier.

Bug: 16208505, 16206748, 16212206
Change-Id: Ieb82a424ca4a92b5674983453c50ba4b695abfb0