Restore API support for Android

[brodycj]

The fork by @zxyang added support for loading the p12 certificate for Android via the API call. Update from the @EbilPanda fork changed the Android implementation in fd7ed1d to use the system certificate keystore (keychain), which seems to be an almost exact copy of johannes-staehlin/cordova-client-cert-authentication.

I think it would have been better to keep both API and system certificate support for Android. This means that this fork should have been based on the @zxyang fork, with additional cherry-pick of commits to add the iOS myp12 file association and Android system certificate capabilities (instead of the ugly merge that I did between unrelated git histories). Or maybe keep the iOS myp12 file association and Android system certificate capabilities in separate plugins.