Add VLOG(collector) to reduce log spam

VLOG(gc) is enabled from AndroidRuntime, using it too much produces
log spam.

Bug: 22858531

(cherry picked from commit a6e1c126299586932ecd3c1133a55a6f8e1107fc)

Change-Id: If60902a8d7855cdabd80208b9b385ce4ba4d9e5f
diff --git a/runtime/base/logging.h b/runtime/base/logging.h
index de46b0c..8aaeaac 100644
--- a/runtime/base/logging.h
+++ b/runtime/base/logging.h
@@ -37,6 +37,7 @@
 // and the "-verbose:" command line argument.
 struct LogVerbosity {
   bool class_linker;  // Enabled with "-verbose:class".
+  bool collector;
   bool compiler;
   bool deopt;
   bool gc;