Add LOCAL_NDK_VERSION.

Used to select between releases of the NDK (r10, r11, etc).

Some of the libraries we use in Android come as prebuilts from
google3, which are most likely built against an older NDK than what
we have in the platform. Since things may not be ABI compatible
release to release, we need to keep legacy versions accessible for
those to keep building.

Change-Id: Ia3df42fa6f3a7bd5108ff6bbb0f2ca06239c3be9