Use previous pressure value if the current fusion data is zero.

Occasionally we'll receive the stylus up signal (pressure = 0) before
we receive the touch screen up signal. Rather than giving pointer a
pressure value of 0 (which is one of the signals of hovering) or
falling back to the touchscreen pressure values (which would make for
an inconsistent stream), use the previous pressure value which should
always be non-zero for a stream of fused data.

Bug: 20449776
Change-Id: I71eb97e7c4ea53e42b0eb54fc1f8ae7f89aad9d1