input: add RELATIVE pointer capture mode

In RELATIVE mode, captured touchpads will behave in the same way as
captured mice, so that apps and games can support them without
additional effort.

Test: make a test app that captures the pointer in RELATIVE mode, and
      check from the InputDispatcher outbound event logs that movements
      on a captured touchpad result in the same format of events being
      sent as movements of a captured mouse. (b/418908873 currently
      prevents this from being verified from the app side.)
Test: $ atest --host inputflinger_tests
Bug: 403531245
Flag: com.android.hardware.input.pointer_capture_modes
Change-Id: I786f19a42660e42fddf78c3385de1bb6e4df97b3