SystemUI: transparency mgr bug fixes

- Avoid potential NPE which crashes SysUI
- Sometimes while in the home screen pulling down a phablet panel would have delayed
    reaction and statusbar would be transparent while the panel is below it, it looks
	ugly and avoid it.

Change-Id: I5e405e34e8dec42ea0fb3dbf00910219b411f202
Signed-off-by: Roman Birg <romanbirg@gmail.com>