Isolate data may need to be deleted on the same thread where it was allocated.
In particular, the task observer set up in the UIDartState ctor must be removed
from the same message loop where it was added.

The engine had been using the same DartIsolate object as the root isolate data
and as the isolate group data.  This object would be deleted when the isolate
group was shut down.  However, group shutdown may occur on a thread associated
with a secondary isolate.  When this happens, cleanup of any state tied to the
root isolate's thread will fail.

This change adds a DartIsolateGroupData object holding state that is common
among all isolates in a group.  DartIsolateGroupData can be deleted on any
thread.

See https://github.com/flutter/flutter/issues/45578