Android: text entry implicit size fits 20 characters

A native unlayouted EditText control fits 20 characters by default.
Let Qt Quick Controls Android Style use the same metrics instead of
adjusting to the textual contents. This gives empty TextField,
SpinBox and ComboBox sensible implicit sizes instead of shrinking
them down to something unusable.

Change-Id: I3cd3948aa3d64daada9d0b7d773cebb6627263f6
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>