b/3307761 Tuning two fingers pan.

User reports jumpy when trying two fingers pan. This change keeps
track of the previous movements of the fingers' middle point,
compares to the change of the fingers' distance, and decides
to pan or zoom or do both.

Change-Id: I8a6a8e9259db85b0e820b6e25ba0822ed289fb45
1 file changed