Eagerly generate local.properties, and always update the flutter.sdk
setting in it, in case FLUTTER_ROOT has changed.

Fixes #8365.
Fixes #9716 - at least the specific issue reported. My Android Studio
still complains about Gradle versions - it ships with v3.2, but requires
v3.3...

Add a 'generate dependencies' task to the Gradle build, which checks if
the snapshot dependencies file exists, and runs an extra build before
the actual FlutterTask if it doesn't. This makes the first build slower,
but sub-sequent builds (without source changes) much faster.

Fixes #9717.