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/device3/Camera3IOStreamBase.h b/services/camera/libcameraservice/device3/Camera3IOStreamBase.h
index abcf2b1..f5727e8 100644
--- a/services/camera/libcameraservice/device3/Camera3IOStreamBase.h
+++ b/services/camera/libcameraservice/device3/Camera3IOStreamBase.h
@@ -84,7 +84,7 @@
 
     virtual size_t   getHandoutInputBufferCountLocked();
 
-    virtual status_t getEndpointUsage(uint32_t *usage) = 0;
+    virtual status_t getEndpointUsage(uint32_t *usage) const = 0;
 
     status_t getBufferPreconditionCheckLocked() const;
     status_t returnBufferPreconditionCheckLocked() const;