ModNavigationBar: don't show search icon hint when ring disabled (4.2+ only)

Revised by bgcngm:
- getSearchLight method from com.android.systemui.statusbar.phone.NavigationBarView
class was only added in SDK 17, so apply only to Android 4.2+
ModPowerMenu: improved option to disable power menu while locked
Won't allow power menu while activity that's allowed to be running
during keyguard lock is in the foreground (e.g. emergency dialer or
camera app)