Filter out unsupported items immediately when updating Taskbar.

Fixes an issue where hotseatItems may not reflect the number of hotseat views in Taskbar. Entries in the array can be null (e.g. no predictions but there is space in the hotseat for them). If these are filtered out immediately, hotseatItems.length will correspond to the number of hotseat views.

Test: go/testedequals
Flag: EXEMPT bugfix
Fix: 379704910
Change-Id: I375d8152fccb6df5a9b783842ecf5dcedf6894c1