CapturedTouchpadEventConverter: report relative axes

In the old touchpad stack, when a touchpad was captured we'd report
AXIS_RELATIVE_(X|Y) values for each finger, as offsets from the location
it had in the previous frame. When I implemented captured mode in the
new stack I didn't realize this, so the new stack introduced in Android
U doesn't report them. This causes problems for games that capture the
mouse pointer and use the relative axes for camera movement.

Bug: 330522990
Test: check the axis values using a test app
Test: $ atest inputflinger_tests:CapturedTouchpadEventConverterTest
Flag: com.android.input.flags.include_relative_axis_values_for_captured_touchpads
Change-Id: I8a1caa4c9315dd41bc712fd9467146bb608cf6f3