[analyzer] Don't collect analytics if not using it; cache `getTopLevelDeclarations` results in DartFixContext

Sometimes - on responding to `edit.getFixes` requests, the method
`getTopLevelDeclarations` is called several times with the same
parameter and caching has then sometimes sped things up.

I then also noticed that it caused `reportAnalysisAnalytics` to be
called many times (e.g. 73 times for one `edit.getFixes` request)
each time spending time collecting data which - on passing to
`analyticsManager.analysisComplete` - was just thrown away because it
only saves the first one.

Combined these changes have been observed to reduce the response time
on `edit.getFixes` by up (down?) to ~20% (e.g. from 5 seconds
to 1 second).

In response to http://b/407797012 where more data is also available.

Unfortunately I haven't been able to reproduce this is a benchmark
(yet anyway). I don't know if it needs slower slower reads, an extreme
amount of files, several contexts or something else entirely.

Change-Id: Id214b0e1b7d8bbef7f6d956408f17fab45618b78
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420323
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>