Android: Support double click event

It's impossible to get the distance between two touch events
down to 5 pixels on e.g. my Nexus 5.

This patch makes it possible to tweak the distance through
the platform theme, and sets the distance to 15% of an inch
on Android. Also provides a way to override the default minimum
of 5 pixels by using an environment variable.

[ChangeLog][Android] Fixed double click events

Task-number: QTBUG-36974
Change-Id: I23d94020c531747d6638b645133611614a2a0703
Reviewed-by: BogDan Vatra <bogdan@kde.org>