Limit dragging using 2 pointers so that it doesn't create a clunky

behaviour when the user uses 2 fingers to resize at once or
sequentalliy.

Test: atest WMShellUnitTests:DragDetectorTest
Flag: EXEMPT fixing bug
Fixes: 336242246
Fixes: 336247472
Change-Id: I9e003b7effd8c7893195afd38aade704e80d969d