xmpp-addr: Backfill missing class method for Java 1.7
This backfills missing class methods for `java.nio.charset.StandardCharsets`
and `java.util.Objects` for compatibility with platforms which do not support
these (mainly Android SDK versions <= 18).
Change `minSdkVersion` to 18, backfill missing methods
This reduces the minimum SDK version to 18 (Android 4.3), which notably is
the last supported version for the BlackBerry OS 10.3 Android compatibility
layer.
Drop support for Android 4.1 and 4.2
