Revert^2 "[metrics] Add StatsdBackend"

Adds a new backend that allows reporting ART's metrics to statsd, where
they can then be uploaded to Westworld.

Manual Testing Instructions:

Run the following commands.

    m statsd_testdrive
    adb shell setprop dalvik.vm.extra-opts              \
                      -Xwrite-metrics-to-statsd\\\      \
                      -Xwrite-metrics-to-log\\\         \
                      -Xmetrics-reporting-period=10
    adb shell stop && adb shell start
    statsd_testdrive 332

After about a minute, you should see several atoms logged from ART.

This reverts commit dbad1ef673140c66cdbcbbf40424674ae8e2b4c8.
Reason for revert: Added statsd apex to prebuilts

Bug: 178236337
Test: Manual, see above
Test: see instructions on https://r.android.com/1591932
Change-Id: I2071983c04c51efe88df6a56c59fc418fe6e9424
diff --git a/libartbase/base/metrics/metrics.h b/libartbase/base/metrics/metrics.h
index 3ea19c8..9345565 100644
--- a/libartbase/base/metrics/metrics.h
+++ b/libartbase/base/metrics/metrics.h
@@ -158,7 +158,7 @@
 
  protected:
   // Called by the metrics reporter to indicate that a new metrics report is starting.
-  virtual void BeginReport(uint64_t timestamp_millis) = 0;
+  virtual void BeginReport(uint64_t timestamp_since_start_ms) = 0;
 
   // Called by the metrics reporter to give the current value of the counter with id counter_type.
   //
@@ -442,6 +442,14 @@
 // Returns a human readable name for the given DatumId.
 std::string DatumName(DatumId datum);
 
+// We also log the thread type for metrics so we can distinguish things that block the UI thread
+// from things that happen on the background thread. This enum keeps track of what thread types we
+// support.
+enum class ThreadType {
+  kMain,
+  kBackground,
+};
+
 }  // namespace metrics
 }  // namespace art