Roboto Font not loaded with --prod build

**Description:**
Roboto Font is not loaded when building with `--prod` flag.

**Steps to Reproduce:**
build app with `ionic cordova run android --prod`
(I don't know if it's the same with iOS)

**Output:**

Error messages:
```
Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-regular.woff Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-regular.ttf Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-medium.woff2 Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-bold.woff2 Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-medium.woff Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-bold.woff Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-medium.ttf Failed to load resource: net::ERR_FILE_NOT_FOUND
file:///android_asset/www/assets/fonts/roboto-bold.ttf Failed to load resource: net::ERR_FILE_NOT_FOUND
```

**My `ionic info`:**
```
cli packages:
@ionic/cli-utils : 1.13.1
ionic (Ionic CLI) : 3.13.2

global packages:
cordova (Cordova CLI) : 7.0.1

local packages:
@ionic/app-scripts : 3.0.1
Cordova Platforms : android 6.2.3 ios 4.4.0
Ionic Framework : ionic-angular 3.7.1

System:
Android SDK Tools : 25.2.5
Node : v8.2.1
npm : 5.4.2
OS : Windows 10

Misc:
backend : legacy
```

Start by reproducing the issue with `ionic cordova run android --prod` and inspect the generated Android assets for the missing Roboto files. Done means the production Android build includes the referenced font files and no longer reports the listed `ERR_FILE_NOT_FOUND` errors.

由索引模型根據 Issue 內容生成。