Mention updating prebuilts version in RenderScriptPrebuilts.md

Mention that during a RenderScript rebase, the prebuilts toolchain used
to build RenderScript tools should also be updated.

Change-Id: I19d456340293861dc0f3b0a17d55dfed8730a2a8
(cherry picked from commit a967394529ae1f039495f1102116c7377f201ef9)
1 file changed