Fix RS NDK usage so we always pull from "current" instead of NDK 8.

The 8 in this definition refers to NDK release 8, not the API level 8.
Setting it to "current" is better, since it still contains the relevant
target API level (9 in our case) for using/building our RS support APIs.

Change-Id: Ib216ea859e137643fba81c39c4b9d0077ec04455