Extract credential storage dialogs to a new activity.

Since now SecuritySettings is a fragment, not an activity, reacting
to intents is not easier anymore. The new activity is transparent,
and it works just like a dialog. This change also refactor'ed
SecuritySettings by moving some preference construction into
security_settings_misc.xml.

Bug: 3120992
Bug: 2117436
Change-Id: I8bd3fc7faa2a31d100b8d67310f9266260cfc31d