Flutter bbclass refactor

-Removes use of flutter bbclass runtime modes
 Installed modes are determined by flutter-engine package config flags.
 Default modes are debug, profile, and release.
-Single json file based on runtime=release. If additional
 parameters are needed this happens as part of workspace
 automation.

Scott Murray:
- update packagegroup-agl-demo-platform-flutter

Bug-AGL: SPEC-4819

Change-Id: I58fc1b30ea3367ec5f47f52b60951cf0be31dcbc
Signed-off-by: Joel Winarske <joel.winarske@gmail.com>
Signed-off-by: Scott Murray <scott.murray@konsulko.com>