Return Key* from getKey

The call getKey can fail. Rather than returning a separate bool that the
caller might ignore, return a pointer to simplify the code.

Bug: 278299254
Test: m libinput_tests && $ANDROID_HOST_OUT/nativetest64/libinput_tests/libinput_tests
Change-Id: I28c25bee8890bdc90ca7e069c803423a7420e6b4