Populate uniq field in uhid.

Sony introduced some logic that combines devices kernel splits into one
Android device based on identical pid, vid and non-empty uniq. Therefore
populate this field with device ID.

We can change that to a separate uniq attribute in register JSON file if
necessary later.

Bug: 38511270
Test: getevent -li shows that uniq is populated.
Change-Id: Ieca0b826c86e9cad7fa3d3c8f98b9cfd54d20c73