SystemUI: Re-adjust signal strength icons

Change-Id: Id6dad0800e5e15ec694b24ecb9a88ccc0d4771d8
core: update power menu reboot drawables

 * also pngcrush'ed with: pngcrush -rem alla -brute -reduce

Change-Id: I9467f00edefce8715c6b2b67c56bd21364e5da56