Camera: Enable request processor during 'onCaptureSessionStart'

Advanced extension implementation can start using the request
processor as soon as they receive 'onCaptureSessionStart'.

Bug: 297173890
Test: atest -c -d cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionSessionTest.java
Change-Id: I9f5fc98c228aa2233c9c9a88053b8849f41acc6e