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We're moving the header/library generation part of the NDK build into
the platform build. A number of reasons for this:

1. Simplifies the NDK API development process by removing the need to
   build the full NDK to test your API. NDK APIs can now be tested by
   CTS the moment they are written rather than needing to wait for a
   new NDK to be dropped into prebuilts/ndk.
2. Allows NDK APIs to be developed in release branches rather than
   AOSP. Right now if we want to avoid announcing an API we have to
   avoid putting it in the NDK until its ready.
3. Keeps the NDK headers and the platform headers from getting out of
   sync. Right now framework headers are only updated when someone
   remembers to update them, and the libc/libm headers were forked
   from bionic years ago, and then again for L.

Test: None
Bug: http://b/27533932
Change-Id: Ie0bf325bba627630d55837ddb3094ea487c41490