Fixing Launcher crases due to methods being called on Ui Threads.

This CL fixes the following two issues:
1. Launcher crashing while calling `addPrivateSpaceSettingsButton` from
   BaseAllAppsAdapter as the thread is UI
2. Decorator is not removed from Private Space Header during reset to
   paused state. https://screenshot.googleplex.com/BZdLqaZQwdDApfV

Flag: ACONFIG com.android.launcher3.Flags.enable_private_space DEVELOPMENT
Bug: 311337961, 311342946
Test: Ran Launcher3 tests
Change-Id: I2763338ebf0f5468ad38f61a781e8a8334194c5c