[Bug Report] el-input not firing input event on android

### Element UI version
2.4.11 (updating to 2.6 doesn't fix)

### OS/Browsers version
Android 9 - Chrome

### Vue version
2.5.16 (updating to 2.6 doesn't fix)

### Reproduction Link
[https://jsfiddle.net/p20gfmos/](https://jsfiddle.net/p20gfmos/)

### Steps to reproduce
1. Use an el-input on a Android device (android 8+) with gBoard keyboard
2. Type something in the input

### What is Expected?
The value of the input is immediately displayed under it in a div.

### What is actually happening?
The `input` instance value is not updated on keypress. It's only updated on blur or if the space key is pressed.

When debugging a bit, I realized the input event wasn't fired by element because `this.isOnComposition` is true in the keypress listener.

I tried on several android phones and simulators and I can reproduce with every Android 8+ devices using gBoard. If I use another keyboard like SwiftKey, it works fine.