Start removing Dependency.get(UiOffloadThread.class)

This actually begins the process of deprecation and removing
UiOffloadThread in favor of using Executors directly.

This CL hits the easiest spots in the code. There are a few
more remaining calls that should be in their own independent CLs.

Bug: 146567476
Test: atest SystemUITests
Change-Id: I2542cc61653f330a1931b15a57e3b7c77d18e2e6