cameraservice: Make getSessionCharacteristics accessible earlier

The current implementation of getSessionCharacteristics forced Camera2
to open a corresponding CameraDevice. However, there is no technical
limitation that prevents getSessionCharacteristics from being
accessible before an openCamera call.

This CL moves the getSessionCharacteristics to be a function in
ICameraService instead of ICameraDeviceUser which allows camera2
to call getSessionCharacteristics before calling openCamera
for a given device.

Bug: 320741775
Test: atest CtsCameraTestCases:CameraDeviceTest passes.
Test: atest CtsCameraTestCases:FeatureCombinationTest passes.
Test: atest CtsCameraTestCases:CameraDeviceSetupTest passes.
Test: atest cameraservice_test passes.
Change-Id: Ia21f882fac5ece59dcd6a70e04b2f81ef6dee4c3