Implement notification grouping on Android

Summary:
This is available starting at API level 20, which is below our minimal
requirement. Grouping can be disabled by the already existing SkipGrouping
flag.

When enabled this puts all notifications from the same eventId in a group,
which makes the display more compact by removing the otherwise duplicated
app name.

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29335