Prevent clearing home between display size changes.

Split display support was previously based on the sizes of supported device profiles. If a device supporting split display increased display size too much, it could trigger a grid migration (due to number of hotseat icons displayed vs in the DB changing as it was no longer seen as split display). The migration would not run however, as the grid had not actually changed, but would still have cleard the DB in preparation of the migration.

By counting the number of supported profiles instead, we can estimate the number of screens invariant of display size changes and avoid undesired grid migrations.

Test: manual
Fix: 187689871
Bug: 187689871
Change-Id: If740c501cab0e80ef6144356ec5618ee30134ed7