Increase thread priority before engine init (#20922)
Increase thread priority before engine init (#20922)

We get huge ANRs in not pure flutter app.
We found that 0.15% of the engine initialization took more than 10 seconds, every 10 million starts.
chinmaygarde/flutter_engine,devoncarew/sky_engine,jason-simmons/sky_engine,flutter/engine,devoncarew/engine,aam/engine,rmacnak-google/engine,rmacnak-google/engine,jason-simmons/flutter_engine,rmacnak-google/engine,devoncarew/sky_engine,flutter/engine,aam/engine,jamesr/flutter_engine,devoncarew/engine,devoncarew/engine,chinmaygarde/sky_engine,flutter/engine,rmacnak-google/engine,chinmaygarde/sky_engine,aam/engine,jamesr/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,flutter/engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,Hixie/sky_engine,aam/engine,devoncarew/sky_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,devoncarew/engine,aam/engine,jason-simmons/flutter_engine,devoncarew/engine,jamesr/sky_engine,flutter/engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,Hixie/sky_engine,flutter/engine,Hixie/sky_engine,jamesr/flutter_engine,jamesr/sky_engine,devoncarew/sky_engine,aam/engine,devoncarew/sky_engine,jason-simmons/sky_engine,flutter/engine,jamesr/flutter_engine,devoncarew/engine,jamesr/sky_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,aam/engine,flutter/engine,jason-simmons/flutter_engine,Hixie/sky_engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,chinmaygarde/sky_engine,aam/engine,rmacnak-google/engine,chinmaygarde/sky_engine,jamesr/flutter_engine,Hixie/sky_engine,jamesr/sky_engine,devoncarew/engine,jamesr/flutter_engine,jason-simmons/sky_engine,jamesr/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine