[all plugins] Conditionalize namespace in build.gradle due to No signature of method: build_[...].android() is applicable for argument types error with AGP <4.2 #125621
Description
We recently added namespace to all of our plugins' build.gradle files to make them compatible with AGP 8.0 (see #125181). I thought I had tested that this would be ignored with older versions of AGP, but it turns out that I only tested with AGP 4.2, and that's actually the first version where namespace works. Developers with AGP 4.1 (which flutter create used in the Flutter 2.x timeframe) and earlier are reporting being broken (#125607, #125605).
This tracks reverting the addition of namespace while we figure out how to move forward without breaking 4.1 and earlier.