Differentiate Uinput devices by productId for testing

Unless we somehow differentiate test Uinput devices, if more than one of
those devices are created, they will be merged together into one
InputDevice in InputReader.

Therefore we use a different productId for each Uinput device class.

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: I5669af54d8457cd572fa5bfbff670e7926237fa4