Camera2: Add prepare for output streams

The prepare call asynchronously pre-allocates buffers for a given
output stream, and then fires the onPrepared callback.

Not implemented for Camera2Device or used in Camera2Client.

Change-Id: I1cccdfff846dd6985133c591dbdceed823929ade
diff --git a/services/camera/libcameraservice/common/Camera2ClientBase.h b/services/camera/libcameraservice/common/Camera2ClientBase.h
index 168ea0a..f1cacdf 100644
--- a/services/camera/libcameraservice/common/Camera2ClientBase.h
+++ b/services/camera/libcameraservice/common/Camera2ClientBase.h
@@ -72,7 +72,7 @@
     virtual void          notifyAutoExposure(uint8_t newState, int triggerId);
     virtual void          notifyAutoWhitebalance(uint8_t newState,
                                                  int triggerId);
-
+    virtual void          notifyPrepared(int streamId);
 
     int                   getCameraId() const;
     const sp<CameraDeviceBase>&