Camera: Avoid possible NPE when forwarding images

Use the local reader argument instead of the member
variable which can get reset.
Additionally, make sure to execute the extension forward
processor 'ImageReader' callback using the extension session
handler.

Bug: 184447319
Test: atest -c
cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionSessionTest.java#testCloseCaptureSession
--iterations=10
atest -c
cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionSessionTest.java

Change-Id: Id8f8818ad478b70438cbd18d46ef79263bc4de9d