Broaden directory & file requirements in validation.

With Android Studio/Gradle's project & flavor structure, a manifest file and src
directory may not necessarily exist. Running android_img_resizer in a specific flavor directory refused to resize the image in my product flavors with this structure:
```
MyProject/
 +-- build.gradle
 +-- src/
 |    |
 |    +-- main/
 |    +-- AndroidManifest.xml
 |    |    +-- java/
 |    |    +-- res/
 |    |    |    +-- drawable/
 |    |    |    +-- drawable-mdpi/
 |    |    |    +-- drawable-hdpi/
 |    |    |    +-- drawable-xhdpi/
 |    |
 |    +-- flavor1/
 |    |    +--res/
 |    |    |    +-- drawable/
 |    |    |    +-- drawable-mdpi/
 |    |    |    +-- drawable-hdpi/
 |    |    |    +-- drawable-xhdpi/
 |    |
 |    +-- flavor2/
 |    |    +-- res/
 |    |    |    +-- drawable/
 |    |    |    +-- drawable-mdpi/
 |    |    |    +-- drawable-hdpi/
 |    |    |    +-- drawable-xhdpi/
```