libbinder: dec refs include count

This is in preparation for changing how/when we decrement refcounts. The
difference in this CL is that the wire format includes a count with an
easy mechanism to set the count we want.

The difference in behavior from this CL is that sometimes, multiple
decStrong refcounts will be combined into one. In the future, we'll want
to combine multiple of these.

Bug: 182940634
Test: binderRpcTest (on host and also on Pixel 3)
Change-Id: I2339a8e0fff3c35ce3c5f562d590da7a12b6670a