Include googletest cmake support.

- Same googletest release (v1.8.0), new root in third_party.
- Update Android.mk and make build (include and file locations).
- Update cmake build (use googletest's cmake support).
- Update gtest include in all tests.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: I5491a2ff3c8869263cd885872c815402236c20b1
diff --git a/test/datarate_test.cc b/test/datarate_test.cc
index ec86604..48be4a4 100644
--- a/test/datarate_test.cc
+++ b/test/datarate_test.cc
@@ -10,7 +10,7 @@
 */
 
 #include "./aom_config.h"
-#include "third_party/googletest/src/include/gtest/gtest.h"
+#include "third_party/googletest/src/googletest/include/gtest/gtest.h"
 #include "test/codec_factory.h"
 #include "test/encode_test_driver.h"
 #include "test/i420_video_source.h"