GN: wrap libperfetto_android_internal in perfetto_build_with_android

Context: chromium roll breakage http://crrev.com/c/2625893 .
I am not sure this will fix the issue but at very least should
make it more obvious. I still cannot find any GN path that would
include android_internal in chromium builds.
Also fix a trailing /  introduced by aosp/1512303 (I don't think
that's the culprit either)

Change-Id: I3aa258f119a3153c9e239fee6f36d34901e67936
diff --git a/BUILD.gn b/BUILD.gn
index 1a97fe9..146ea00 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -63,7 +63,7 @@
 }
 
 if (perfetto_build_with_android) {
-  all_targets += [ "src/android_internal/:libperfetto_android_internal" ]
+  all_targets += [ "src/android_internal:libperfetto_android_internal" ]
 }
 
 if (enable_perfetto_tools) {