Make browser preference fragment valid

Subclasses of PreferenceActivity must override isValidFragment(String)
to verify that the Fragment class is valid. But
com.android.browser.BrowserPreferencesPage has not checked this.

Change-Id: I3650eff8a6d0547f14c6aff54425204d6c19f103

Conflicts:
	src/com/android/browser/BrowserPreferencesPage.java