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/Camera3DummyStream.cpp b/services/camera/libcameraservice/device3/Camera3DummyStream.cpp
index 01edfff..ecb8ac8 100644
--- a/services/camera/libcameraservice/device3/Camera3DummyStream.cpp
+++ b/services/camera/libcameraservice/device3/Camera3DummyStream.cpp
@@ -87,7 +87,7 @@
     return OK;
 }
 
-status_t Camera3DummyStream::getEndpointUsage(uint32_t *usage) {
+status_t Camera3DummyStream::getEndpointUsage(uint32_t *usage) const {
     *usage = DUMMY_USAGE;
     return OK;
 }