Improvement: Add AndroidX Support

[LordParsley]

• @yashpalraj upgrade Android support libs to v28.0.0.
• Remove calls to deprecated setOnTabSelectedListener in favour of addOnTabSelectedListener.
• Fix trailing category end tag in demo manifest.
• Add using the local library for release flavour as checks like lintVitalRelease fail (e.g. https://github.com/edbort/Android-ContactPicker/commit/e92b67f6197f9df9db99c747d3394c7dfb5a88c1 for EXTRA_WITH_GROUP_TAB).
• Upgrade Android Gradle Plugin to v3.5.0-rc01 & gradle wrapper to v5.4.1-all.
• Migrate to AndroidX.