Ignore hover events while pointers are already down

If pointers are already down from another device, finish the current
gesture and ignore events from other devices.

When multi-device support is enabled, we can remove this restriction.

Bug: 268538505
Test: m inputflinger_tests && $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: I6419b6d118760d285457bc9e5127e055f21ecf7f