Support arbitrary loggers.

While the defaults (logd or stderr) make sense for most use cases,
there are places that can only log to the kernel, or need to log to a
file, etc.

Allow the user to pass in an arbitrary logging object, and provide
LogdLogger and StderrLogger as defaults.

Change-Id: I62368acc795ff313242bb205d65017404bf64e88
diff --git a/base/test_main.cpp b/base/test_main.cpp
index c49ca4b..546923d 100644
--- a/base/test_main.cpp
+++ b/base/test_main.cpp
@@ -20,6 +20,6 @@
 
 int main(int argc, char** argv) {
   ::testing::InitGoogleTest(&argc, argv);
-  android::base::InitLogging(argv);
+  android::base::InitLogging(argv, android::base::StderrLogger);
   return RUN_ALL_TESTS();
 }