NoSuchElementException handing in SystemRequestObserver

- Added logging to SystemRequestObserver
- unlinkToDeath no longer called if requested modes are empty
- catching NoSuchElementException, if binder died, howerver callback is not called

Bug: b/377725662
Test: atest SystemRequestObserverTest
Flag: EXEMPT trivial bugfix
Change-Id: Icce0c3894e601ab1209201fbcf6752f69b0b1278