Construct classloader for apps with no java code

The commit 6f06ad7d
fixed a way to load native library for NativeActivity
by using classloader logic to find the library; which
also fixed the problem of native activity not working
when library is supposed to be opened directly from
the apk.

As a side effect it introduced regression - apps without
java code did not have correctly initialized classloader.
This change fixes this by constructing classloader with
empty dexpath but valid nativeLibrarySearchPath.

Bug: http://b/26015561
Bug: http://b/27250344
Change-Id: I50f1119f0976a95edd75d88efb9fcdedc57efbc3