Modified "Empty button" > "Leave empty"

"Empty button" implies a placeholder button, which could be confusing to new users.

Change-Id: I092bdb0c8d3cfad79560526f383cdb7efb730a06
Merge "Modified "Empty button" > "Leave empty"" into cm-11.0
Merge branch 'cm-11.0' of github.com:CyanogenMod/android_frameworks_base into cm-11.0