Fix the error on project AppWithSetting

[nrohmen]

Hi, i am Nur Rohman. I am the trainer of Android Developer Fundamental Training for faculty in Indonesia. I have replaced PreferenceManager on MainActivity to android.support.v7.preference.PreferenceManager, because it causes errors.

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[nrohmen]

I signed it!

[googlebot]

CLAs look good, thanks!

[jutikorn]

@ennur I think we can change this line from:
import android.preference.PreferenceManager;

to

import android.support.v7.preference.PreferenceManager;