Use Gradle with --offline when building the browser part

boklm noticed in comment:23:ticket:27977 that we are using Gradle's --offline mode for building Orbot which fails hard if the dependencies are not available (at least it seems so; I have not checked). Furthermore, he suggested we use that mode as well for the Firefox build part. This seems like a good defense in depth given bugs like legacy/trac#28752 (moved).