Move express prescaling to DecisionSummaryMaker and monitor express rates after prescale

Moving express prescale calculation upstream, which resolves a few issues / feature requests.

1. Move express prescale calculation (call to IPrescalingTool) from StreamTagMakerTool to DecisionSummaryMakerAlg and write the prescale decisions to the HLT_NavSummary container (the writing was an outstanding to-do from ATR-20870).
2. Read the express prescale decisions in StreamTagMakerTool from HLT_NavSummary instead of calculating them.
3. Read the express prescale decisions in TrigSignatureMoni in order to properly monitor express stream rate and counts after the express prescale, fixes ATR-25616.
4. (Unrelated, but done since already touching this code) Move StreamTagMakerTool filling of m_mapping from initialize() to start() and clear the map before filling, fixes ATR-21173.