Support querying active record sources

Add support for querying whether there is currently a recording
 underway from the specified audio source.

Bug 7314859

Change-Id: I270f27eff4dcf1cc179089859a4c690140825ad1

include/hardware/audio_policy.h

@@ -232,6 +232,9 @@ struct audio_policy {
                              audio_stream_type_t stream,
                              uint32_t in_past_ms);
 
+    bool (*is_source_active)(const struct audio_policy *pol,
+                             audio_source_t source);
+
     /* dump state */
     int (*dump)(const struct audio_policy *pol, int fd);
 };