TouchpadInputMapper: send hover events after UP

After an UP event, it's expected that an input device with a cursor
(e.g. mouse, touchpad, stylus) sends a HOVER_ENTER event so that the app
knows the cursor is hovering again. From how CursorInputMapper does
this, it seems that the convention is for the mapper to send a
HOVER_MOVE which is then turned into HOVER_ENTER later in the stack.

This is an area we'd like to refactor in future, but for now this fixes
the issue in a way that's consistent with CursorInputMapper.

Bug: 287246827
Test: in a test app, click with a touchpad and check the UP event is
      followed by a HOVER_ENTER
Test: atest inputflinger_tests:GestureConverterTest
Change-Id: I990428552549b3bc8a1eda7199dcbe92801ca4e1