build: Update Gradle [wrapper] and Android Studio Gradle plugin.

This also improves the Groovy build.gradle style.
Album: Remove unused constructors.
Album: Order signatures the same way.
Item: Clean up warnings
Artist: Remove unused value albumCount.

It was probably disabled because it was a bad idea.
MPDAsyncWorker: Remove MPD null checks.

By the time the MPD object gets in MPDASyncWorker it can no longer be null.
item: Remove info(), it's unused and needs maintenance.
SettingsHelper: Cleanup warnings.
MPDConnection: Throw IllegalStateException on null getHostPort/getHostAddress.

This should have been a RuntimeException in the first place.