Add a way to use libraries from the Android tree

This patch adds a gn variable build_with_android which makes it possible
to use Android internal (i.e., non-NDK) libraries when Perfetto is being
built as a part of the Android tree.

Change-Id: I935582fa969ab230b9ab0ac7d392dfeace802b0e
diff --git a/BUILD.gn b/BUILD.gn
index 512934e..1996c66 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -66,6 +66,10 @@
       "src/traced/probes",
       "src/traced/service",
     ]
+    if (build_with_android) {
+      cflags = [ "-DPERFETTO_BUILD_WITH_ANDROID" ]
+      libs = [ "utils" ]
+    }
   }
 
   # The unprivileged trace daemon that listens for Producer and Consumer