Streamline RootTaskInfo creation for testing

newRootTaskInfo wraps up a lot of boilerplate and has sensible
defaults to make usage a bit more compact. This is preparation
for expanding tests to cover more code and more situations.

This change refactors existing usage in the previous work
profile policy code tests.

Bug: 327613051
Flag: NONE
Test: atest ScreenshotPolicyImplTest
Change-Id: I917b005aa76a9821c368f64c960f4989dc0bdd19