Don't include Roboto fonts in Gradle builds. (#8169)

The "old" APK build did not include Roboto fonts, but the new
Gradle-based build did. This is due to `flutter build flx` defaulting to
include the fonts, where the old `flutter build apk` defaulted to NOT
include them.

So let's change the Gradle build to also not include Roboto fonts.

Fixes #8149