Update Glide to v4.8 or later

The version of Glide we're using right now is 4.6.1, but we need to update to v4.8 or later in order to get AndroidX support. For WCAndroid we upgraded to v4.9.0 and recommend the same for WPAndroid.

Ref: https://github.com/bumptech/glide/releases/tag/v4.8.0