Add support for having more hotseat icons in the DB than we show

Split InvariantDeviceProfile#numHotseatIcons into two variables:
numDatabaseHotseatIcons and numShownHotseatIcons. These are generally
the same, but different DisplayOptions within the same GridOption
can choose to show different numbers of hotseat icons while sharing
the same database.

numDatabaseHotseatIcons is used for all reading/writing/migrating
purposes, while numShownHotseatIcons determines how many Hotseat
icons to show in the UI.

Test: Existing tests pass, added two new migration tests
Bug: 184789479
Bug: 171917176

Change-Id: I54583504f61a47a4444b6a637ebb7e3ab31528b7