Always use signed leb128 encoding

 According to runs on /system/lib there using
 unsigned leb128 does not save us any additional
 space. In order to keep packing as simple as
 possible switch to using signed leb128 for
 everything.

Bug: http://b/18051137
Change-Id: I1a47cb9eb2175895b3c3f7c13b4c6b1060de86c0