Android/SwitchStyle: use TextMetrics instead of FontMetrics

Ensures that the thumb text width is correctly calculated regardless
of QML's arbitrary binding evaluation order.

Change-Id: I71526db963fec6d54ad717f46382537591268e37
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>