Enable network during compile for Flutter app recipes

Enable network during compile for Flutter app recipes

Explicitly enable network access during compilation in the Flutter
app recipes for now to avoid hangs when building on systems where
BitBake's network sandboxing is working.  The underlying issue has
been raised with upstream, but may take a while to resolve, so we
need to work around it in the meantime.

Bug-AGL: SPEC-5215

Change-Id: I8cd9cad626addd85ff77a22e221a817b09f0e73f
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30152
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>