Adjust nativeLibraryDir for package contexts of multiArch installs.

When we're creating a package context for a multi-arch app,
adjust the native library directory to match the bitness of
the process creating the context.

This change also removes apkRoot (which wasn't really being used)
and replaces it with a fully constructed secondary library path.
The secondary library path is a transitional measure until we
can reorganize the system image so that we can use nativeLibraryRoot
for system paths as well (nativeLibraryRootRequiresIsa will then
be true for all packages except for legacy installs).

bug: 16013931

Change-Id: I5ae090334b377b9e087aecf40075fab81b20b132