Use nativeloader to load NativeActivity

System apps are the exception and they can
access internal platform libraries (this
is needed for bundled apps to work correctly).

Also fix the way NativeActivity loads the native
library to correspond to the way BaseDexClassloader
does it.

Bug: http://b/22548808
Bub: http://b/25777936
Change-Id: Idc94cdded182ea2cb1cbebc76c336cc3394c7ebe