SystemUI: fix calculating alpha for back button in landscape mode.

Change-Id: Iea7a2eed86d4b65d3772b95f613d1853c66a0158
Merge "SystemUI: fix calculating alpha for back button in landscape mode." into cm-11.0
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_frameworks_base into temp