Try to deduplicate observer iteration code

Later on, registerObserver() can take a handler argument and
invokeForAllObservers() can be updated to post to it when
provided.

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net passes
Bug: 32163131
Change-Id: Ibc32079e1084e2e43f7826fe6b2bdbb83c482e26