Add SNPE deps for caffe2 benchmark android binary

Summary:
Adding snpe dependencies to caffe2_benchmark so that this can benchmark SNPE models on portal devices.

Also need to change ndk_libcxx to gnustl till snpe is updated to work with ndk.

Test Plan: Tested on top of the stack.

Reviewed By: linbinyu

Differential Revision: D23569397

fbshipit-source-id: a6281832804ed4fbb5a8406f436caeae1ff4fd2b

Author: agarla

mode/aibench_caffe2_android

@@ -1,3 +1,3 @@
 --config
 caffe2.strip_glog=0
-@fbsource//fbandroid/mode/ndk_libcxx
+@fbsource//fbandroid/mode/gnustl