Remove prebuilt static library "libthreaded_app.a".

Remove prebuilt static library "libthreaded_app.a".

Make the "glue library" part of the NDK as an importable module,
this has several benefits:

- no need to distribute a binary here with no easy way to regenerate it

- no need to explicitely list -lthreaded_app in your LOCAL_LDLIBS
  (this is handled automatically by the module import capability)

- allows easier native debugging of what's really happening.

Note that the header is renamed <threaded_native_app.h>

+ Modify the native-activity sample to use and import the new module
+ Start documenting usage in the header file. We probably need something
  better, and will probably put it under development/ndk/docs/ at some
  point.

After this patch, we should be able to get rid of the code under
framework/base/native/{include.glue}

Change-Id: I6e81d70a225a6ca006beabf6e8b42529e8f371b9