Debounce details screen navigation to prevent crash - #515

Rapidly tapping a package list item (e.g. a dependency shown on the
details screen itself) queued fragment transactions faster than their
postponed enter transitions could finish, leaving FragmentManager in
an inconsistent state and crashing with:

  IllegalStateException: Fragment DetailsScreen{...} did not return a
  View from onCreateView() or this was called before onCreateView().

Ignore repeated navigation to the details screen within 500ms of the
previous one so only one transition is ever in flight.

Fixes GrapheneOS#436