Settings: Forward port Change behavior when back pressed in PerformanceSettings

When back key pressed, finish this activity if user does NOT accept 'Ok'
in warning dialog rather than show it.
Id: Ied763db4f76a3a2a36f134bd93a0f18047c6cf5c

Change-Id: I07d41d43d00e4fb42ad526d27ee9188d830906ea