Load RootInfo in background, invalidation.

Move all RootInfo queries to background threads to avoid janking
the UI.  Update passes happen on spawed task, which swaps out updated
cache results when finished.  Support partial updates when only a
single package/authority has changed.  Watch for change notifications
for roots, since flags can change over time.

Ignore stopped packages when in background, but query them for roots
when launching any picker UI.

Optimize management launches by treating as one-shot requests that
don't need to wait for all RootInfo.

Bug: 10600454, 10745490
Change-Id: Ibc7b15688ef6b41bd7e9dd0d7564b501e60e49a9