Don't build trace_processor:unittests on Android

Minor follow ups to "Push cpu first in FtraceEventBundle"
- Adjust a comment
- Don't build trace_processor:unittests on Android

Change-Id: I0dc352da23d0db51b612959b50d3f2920ecae4d2
diff --git a/BUILD.gn b/BUILD.gn
index dd03eaf..b6738d7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -115,7 +115,7 @@
       "tools/sanitizers_unittests",
     ]
   }
-  if (build_standalone) {
+  if (build_standalone && !is_android) {
     deps += [ "src/trace_processor:unittests" ]
   }
 }