Add clear_system_cache to android_tools

This is to later use clear_system_cache as a standalone tool for
startup benchmarking on Android.

Bug: 760498
Change-Id: Ifd15e65158742d0876a2209b87e4d30e485f84f6
Reviewed-on: https://chromium-review.googlesource.com/776883Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517809}

tools/android/BUILD.gn

@@ -17,6 +17,7 @@ group("android_tools") {
     "//tools/android/md5sum",
     "//tools/android/memtrack_helper:memtrack_helper",
     "//tools/perf:run_benchmark_wrapper",
+    "//tools/perf/clear_system_cache",
   ]
 }