Drop support for Android 4.3

[webxoss]

Can newest chrome be installed on Android 4.3?

If dropped, we can use features like complete flex box, vh, calc().

[Deardrops]

in my view, when we finally put out webxoss-next, Andriod 4.3 drives have died down.
so it is acceptable to drop support for Andriod 4.3 .