Create SystemApi for statsd ready broadcast

The statsd ready broadcast needs to be sent from statsd apex with
Intent.FLAG_RECEIVER_INCLUDE_BACKGROUND flag that is @hide and cannot be
promoted to @SystemApi.
In R, this was worked around by getting the list of registered broadcast
receivers and sending them directed broadcasts to wake them up in
ag/10404099.
However, this is inefficient if there are multiple receivers so this
change creates a SystemApi for sending the actual broadcast with the
flag as recommended in ag/10404099 after it was submitted.

Bug: 150746848
Test: m
Test: flash
Test: ensure broadcast is received via logcat
CTS-Coverage-Bug: 184014462

Change-Id: I5103d8c6e87bdb47930cb7457731050da3d8e566
diff --git a/services/api/current.txt b/services/api/current.txt
index a3e6715..a0b1e33 100644
--- a/services/api/current.txt
+++ b/services/api/current.txt
@@ -105,6 +105,14 @@
 
 }
 
+package com.android.server.stats {
+
+  public final class StatsHelper {
+    method public static void sendStatsdReadyBroadcast(@NonNull android.content.Context);
+  }
+
+}
+
 package com.android.server.wifi {
 
   public class SupplicantManager {