In the visual studio community edition 2017, sometimes the selection of Supported ABIs from Android Options wont work.
In that case please verify that the .csproj has the following line and no duplicate lines in the same build configurations.
<AndroidSupportedAbis>armeabi;armeabi-v7a;x86;x86_64;arm64-v8a</AndroidSupportedAbis>
In order to edit,