Tests for ContextMenu in FormEntryActivity

#### Problem description
We need tests to make sure that ContextMenu for every widget works as expected to avoid problems like:
https://github.com/getodk/collect/issues/3751
https://github.com/getodk/collect/issues/3761

as we agreed in https://github.com/getodk/collect/pull/3770#issuecomment-617105039 ideally those shouldn't be UI test.

Cases we should tests:
- removing answers if there is one answer on a screen (for every single widget)
- removing answers where there are multiple answers on a screen to cover https://github.com/getodk/collect/issues/3751
- asserting that for every widget `registerToClearAnswerOnLongPress()` is called just once to cover https://github.com/getodk/collect/issues/3761

Start at FormEntryActivity and the ContextMenu code, then trace each widget's registerToClearAnswerOnLongPress() call. Add non-UI tests for clearing one and multiple answers across widgets and for ensuring each registration happens once. Done means the cases described in the issue are covered without relying on UI tests.

Written by the indexing model from the issue text.