[Bug & Workaround] Command 'ionic cordova build android --prod --release' not working

**Ionic version:** (check one with "x")
(For Ionic 1.x issues, please use https://github.com/ionic-team/ionic-v1)
[ ] **2.x**
[ x] **3.x**
[ ] **4.x**

**I'm submitting a ...** (check one with "x")
[ x] bug report
[ ] feature request

**Current behavior:**

ionic cordova build android --prod --release
This command will not copy all fonts to platforms\android\app\src\main\assets\www\assets\fonts
So when you execute the apk in the phone you will get a blank screen

**Expected behavior:**

Get the apk working intead a blank screen

**WORKAROUND**
Create the apk in 2 steps:
ionic build --prod --release
cordova build android --release

**Other information:**

**Ionic info:** (run `ionic info` from a terminal/cmd prompt and paste output below):

```
cli packages: (C:\Users\Julian.JULIANPC\AppData\Roaming\npm\node_modules)
@ionic/cli-utils : 1.19.2
ionic (Ionic CLI) : 3.20.0
global packages:
cordova (Cordova CLI) : 8.0.0
local packages:
@ionic/app-scripts : 3.1.8
Cordova Platforms : android 7.0.0 ios 4.5.4
Ionic Framework : ionic-angular 3.9.2
System:
Android SDK Tools : 26.1.1
Node : v9.9.0
npm : 5.8.0
OS : Windows 10
Environment Variables:
ANDROID_HOME : D:\android\android-sdk
Misc:
backend : pro
```

首先，使用回報的 Ionic CLI、Cordova、Android 和 Windows 版本重現 `ionic cordova build android --prod --release`，然後將其輸出與兩步驟 workaround 進行比較。追蹤該命令的資產複製與發布建置入口點；當所有字型都到達 Android 資產目錄，且產生的 APK 能夠開啟而不顯示空白畫面時，即表示完成。

由索引模型根據 Issue 內容生成。