bug: www/ folder isn't cleared when doing a release build, causing a huge apk.

# Bug Report

**Ionic version:**
[x] **4.3**

**Current behavior:**
When executing a release build after a debug build, the `www/` folder is not cleared, and the release build will have debug chunks and sourcemap files inside the apk. This causes a very big `.apk`.

**Expected behavior:**
The `www/` folder should be cleared before performing a release build.

**Steps to reproduce:**
1. `ionic cordova build android`
2. `ionic cordova build android --prod --release`

Without a debug build:

After a debug build:

Extracted .apk:

**Workaround**

Execute `rm -rf ./www/` before building a release build.

**Ionic info:**

```
Ionic:

ionic (Ionic CLI) : 4.12.0 (C:\Users\freek\AppData\Roaming\nvm\v10.15.0\node_modules\ionic)
Ionic Framework : @ionic/angular 4.3.0
@angular-devkit/build-angular : 0.13.8
@angular-devkit/schematics : 7.3.8
@angular/cli : 7.3.8
@ionic/angular-toolkit : 1.5.1

Cordova:

cordova (Cordova CLI) : 9.0.0 (cordova-lib@9.0.1)
Cordova Platforms : android 8.0.0
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.3, cordova-plugin-ionic-webview 3.1.1, (and 10 other plugins)

System:

Android SDK Tools : 26.1.1 (C:\Users\freek\AppData\Local\Android\Sdk)
NodeJS : v10.15.0 (C:\program files\nodejs\node.exe)
npm : 6.4.1
OS : Windows 10```

Reproduce the issue with `ionic cordova build android` followed by `ionic cordova build android --prod --release`, then trace the CLI's release-build entry point and its handling of the `www/` folder. Done means a release build no longer includes debug chunks or sourcemaps left by the preceding debug build, while the documented workaround is no longer needed.

由索引模型根據 Issue 內容生成。