Update AGP & Kotlin version, use AndroidX, fix sample app, make stories progress view resizable, enable resizable gaps & rounded corners

[joetimmins]

This PR will update:

• Android Gradle Plugin -> 4.0.1
• Gradle -> 6.6
• Kotlin -> 1.4.10
• Android Support Library -> AndroidX
• Sample app -> now uses library in project instead of 1.1.2

This PR fixes:

• Sample app: No findViewById call to set up stories progress view, was causing the stories progress view to not appear
• StoriesProgressView: Now respects layout_height, all child views are set to match_parent, allows library users to change height of progress bar

This PR adds:

• StoriesProgressView: progressGap argument added, allowing library users to change the width of the gap between progress bars, fixes how to modify spacing between progress view? #8
• StoriesProgressView: cornerRadius argument added, allowing library users to round the corners of the progress bars, fixes round edges of progress #10

In order to add the rounded corners, the progress bar views now have a MaterialShapeDrawable placed on them. This requires the Material Components dependency, which has been added to the library module.