Fixed a bug that IllegalArgumentException occurs in SystemUI

This problem occurs when DEBUG of BarTransitions.java is true.

When initializing the SystemUI, it was implemented to specify -1 to oldMode.
modeToString does not assume that the -1 is set to oldMode.
Therefore, IllegalArgumentException occurs.

Change-Id: Icd15d0d1b041b3126d37677dfe894d0a02c22c18