fb: small cleanup and fixes in navbar transitions

this commit fixes several things

- cleanup: no need to go every time trough all calculations if back
button is not present

- force to set alpha value for backbutton on disableflag
changes (merge derp from 4.4.1 merge)

- new introduced workaround from google to animate the altbackkey->backkey
on password lockscreen should not depend on home button view existence and
should only call for the respective disable flags which are always present
from the system. As well which is needed for our navbar call the animation
values from whatever button is existend which behaves like home (so all buttons
only recents and back button not)

whatever still to do if the user uses password as keyguard method
google introduced a workaround to give the altbackicon->backicon
same transition then all other icons....but this workaround is not
really good working and has off timings compared to the other buttons.
Will be fixed in a seperate patchset and should be submitted back to
google as well.

Change-Id: I2c3135b7eb3b87042b757c422f77ef9f5ed280a3