Cast away signedness of built-ins in BitSet.

Most of the built-in functions return signed values but BitSet deals
only in unsigned values. We can safely cast these away though as they're
always indices, which can't be negative.

Bug: 160010896
Test: atest inputflinger_tests, atest libinput_tests
Change-Id: I6e0b33972fabcd41ad1c269ea0e2a07b13b33c12