Fixing CancellationSignal and ProcessingSignal

The signal remote instances need to be fetched from remote
implementation side to allow covering cases where signal might have been
called before /during and after the signal is handed over to the
implementation.

Context: go/isolated-aicore
Bug: 329418533
Test: CTS
Change-Id: Ic796a5f3233f421cd631592134fedcdd74480778