Fixes #137600 
Protect flutter analyze --suggestions from null error when AGP value is missing
Update template with a reference to new agp definition location 
Look for AGP version being set in settings.gradle (change to templates happened in https://github.com/flutter/flutter/commit/dbe0ccd8853dabdcb0f94ce2cdef892a1b10e48a#diff-20537fb84ee37894a3f3d9723a06bcf2674290ee25aa83332c2524a1f7546a6d