Avoid Cleartext HTTP traffic not permitted on Android P

[paolorotolo]

This PR fixes the "Cleartext HTTP traffic not permitted" on Android P: https://developer.android.com/training/articles/security-config#CleartextTrafficPermitted

Actually 2 fixes are available:

• continue using HTTP APIs and allowing cleartext traffic in Andorid Manifest with:

android:usesCleartextTraffic="true"

• Switch to HTTPS APIs.

    private const val BASE_POSTER_PATH = "https://image.tmdb.org/t/p/w342"
    private const val BASE_BACKDROP_PATH = "https://image.tmdb.org/t/p/w780"
    private const val YOUTUBE_VIDEO_URL = "https://www.youtube.com/watch?v="
    private const val YOUTUBE_THUMBNAIL_URL = "https://img.youtube.com/vi/"

This PR implements the second option.

Please review and merge.

[skydoves]

@paolorotolo
Thank you for your PR!!