Add ShellDesktopThread

This CL extracts the ShellDesktopThread and related changes from
ag/30771569 to enable faster integration.
This thread will be used for visual indicators in desktop mode as part
of the new system UI threading model to reduce jank.

Bug: 366413536
Test: build
Flag: EXEMPT adding new unused thread, no functional changes
Change-Id: Id6484194485bd88a47302078ae796b8fab79ceaf