Content model: fix keyboard delete issue on Android

[Rain-Zheng]

Some Android IMEs (such as Gboard and Samsung keyboard) don't support KeyboardEvent well. The key property of keyboard events will always be Unidentified when using these IMEs. To workaround this issue, we need to handle InputEvent instead.

The PR changes the DOM event handling logic in EditPlugin:

1. When receiving Unidentified keyboard event on Android, check the next beforeinput event.
2. Create a keyboard event with correct key according to the type of input event: deleteContentBackward corresponds to Backspace key, and deleteContentForward corresponds to Delete key.
3. Use the created keyboard event to trigger keyboardDelete.