Use ImeWindowVisibility and BackDispositionMode

This creates an IntDef for imeWindowVis values, as well as making sure
this and the BackDispositionMode are present whenever these values are
used/passed, to ensure validation and provide a form of documentation.

Flag: EXEMPT refactor
Bug: 358587238
Test: presubmit
Change-Id: Ia9fe2a1964dc02d002554c50be3779a732f23670