Move handling mojom::BatAdsClient from UI thread

[atuchin-m]

As investigated in #41549 decoding DBTransactionInfo mojo messages happens on UI thread.
In some real-world scenarios it could takes 30ms+ even on high-perf m1 machines resulting in hanging UI thread.
That code need to be move from UI or replace to a one without mojom-encoding/decoding (as @aseren suggested in the thread).

slack thread: https://bravesoftware.slack.com/archives/C3T9S9WKD/p1728567272430889

[iefremov]

@tmancey perhaps this could be P2 - the user impact is pretty visible (so i assume this not merely dev concern, but a product bug, when reproduced)

[bridiver]

Why are we returning such a large amount of data from the DB in the first place vs paginating the results or doing aggregations in SQL?

[tmancey]

@iefremov because this to my knowledge only occurred after a recent migration path, we added as a P3 which will be fixed before we bump the database version again. If this has changed we can move to a P2.

[tmancey]

I will run some perf tests locally using chrome://tracing. Thanks

[atuchin-m]

The trace from the perf pre test when we have this long UI task.
tmplc3ac0yr.pb.gz