Allow values from SourceClass inside rust Source

Inside ViewTest::testOnTouchEventScroll, some MotionEvents are created
without a valid source. During injection, they get appended with
SourceClass::Pointer.

If these events are sent into the InputVerifier, it's basically being
asked to convert 0x2 === AINPUT_SOURCE_CLASS_POINTER into rust's
input::Source.

This fails during unwrap:

$ TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
[ RUN      ] InputVerifierTest.BadSourceProcess
thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', frameworks/native/libs/input/rust/lib.rs:84:35
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5
Aborted

To mitigate this, add the SourceClass definitions into Source, thus
allowing such conversions.

Bug: 303143553
Bug: 211379801
Test: TEST=libinput_rust_test; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Test: TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I46415d88251937959104bb82a3976e72ffcfcaf9