liblog: Assure shifting behavior is consistent

Shifting a signed 32-bit value by 31 bits is implementation-
defined behavior.  So we change to an unsigned value for our
shift by 31 bits, and go ahead and change the others to
unsigned for consistency.

Test: TreeHugger
Change-Id: Ib98f9b1e468d28dafd09e86273bf76beb1ea1fa5