TextArea: let style specify the default text margin

The hard coded 4px margin is unsuitable especially on High DPI
devices. For example on Nexus 5 it should be 36px, so let the
Android style specify a suitable value.

Change-Id: I31287d2e78ae822b8bd63ea19674c85b08975426
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>