Retire LOCAL_NDK_VERSION.

We are now unifying the NDK versions of unbundled native code to always
use the latest NDK.
We don't need the variable LOCAL_NDK_VERSION now.
To build native code with NDK, you need set just LOCAL_SDK_VERSION.

Bug: 6932421
Change-Id: I86f05a264249cda6bae97b4b1616f03700cd9dfa