Worked around back button navigation issue

Calling startWithFragmentAsUser() without specifying FLAG_ACTIVITY_NEW_TASK to
the intent starting the fragment could cause a native stack corruption. See
b/17523189. However, adding that flag and start the preference panel with the
same UserHandler will make it impossible to use back button to return to the
previous screen. See b/20042570.

We work around this issue by adding FLAG_ACTIVITY_NEW_TASK to the intent, while
doing another check here to call startPreferencePanel() instead of
startWithFragmentAsUser() when we're calling it as the same user.

Bug: 20042570
Change-Id: I26b269414f410912b77aaa553a3fccebfa148659