Add more utitlities to ThreadFactory.

Add the ability to create Handlers in TheadFactory, as well
as the ability to create Executors from Handlers.

Use these in ScreenDecorations. Also, use the executor rather than
then handler in ScreenDecorations where possible.

Bug: 177323724
Test: atest SystemUITests
Change-Id: Icaedbfe3dc89f5bbc00c780533cb46279c2d2088