This makes the processing of RawKeyboardEventData.getModifierSide consistent among the platforms.

Before this change, some platforms (Android) didn't handle the case where the "either" flag was set, but no side flag was set (e.g. "shift" was set, but not "shift left"), and instead said that no keys were down by returning null (which was wrong).

Some platforms (Linux, Windows) also returned KeyboardSide.any from getModifierSide, when the contract says that it will never return KeyboardSide.any. Those methods now return KeyboardSide.all in the case where no side is specified, as macOS and Fuchsia already did.

Now, all platforms will return KeyboardSide.all from getModifierSide when it's not clear which side the key was on.