Phone: correct summary of national roaming [translation]

According to Android's writing guidelines for checkbox
settings [1], the summary should not reflect the state
of the checkbox setting but should describe the action
itself.

A workaround was applied earlier on, but this completely
removes the extra summary.

[1] http://developer.android.com/design/patterns/settings.html#writing

Change-Id: Ie2f6df479576d571d6402daa998e808f47e3aaaf