CB-7980 Set BUILD_MULTIPLE_APKS only if webview != system

When using system webview, there is no need to build multiple APKs.

See https://issues.apache.org/jira/browse/CB-7980
CB-7980 Set min/target SDK version via manifest

minSdkVersion was previously set in post-prepare.
Now it's copied from manifest to config.xml in pre-prepare.
cordova-lib then copies it from config.xml to AndroidManifest.xml