Measure time to first valid preview frame in testImageCapture.
Bug: 11999851

Change-Id: Iec695d7de98454435fc9ae239a500fedb9eee894
diff --git a/proguard.flags b/proguard.flags
index 198df75..624af0f 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -14,6 +14,7 @@
   public long getPictureDisplayedToJpegCallbackTime();
   public long getJpegCallbackFinishTime();
   public long getCaptureStartTime();
+  public long getFirstPreviewTime();
 }
 
 -keep class com.android.camera.VideoModule {