Add TaskbarControllers#runAfterInit()

And use it in TaskbarModelCallbacks#commitItemsToUI(), which in rare cases can run during init() before other controllers (namely TaskbarStashController) have been initialized.

Test: call commitItemsToUI() in TaskbarModelCallbacks#init(), ensure no crash
Fixes: 204847702
Change-Id: Ib9df3ea91d41c76ad1d3392e43b444b8b4a91374
Merged-In: Ib9df3ea91d41c76ad1d3392e43b444b8b4a91374