unlockDialog: also center the time in 12h format

Since the time string uses a space to pad single-digit numbers, the centered label looks off-center. Fix this by applying the trim() function to get rid of whitespaces. Here's comparison of how it looked before and after this commit.

Before	After

The drawback of this approach is that the label will change position slightly when the time changes, but for the lockscreen that's okay.

Fixes #6193 (closed)