Use vector drawables for the SimpleExoPlayer player control buttons.

[colinrtwhite]

This is a simple pull request to update the SimpleExoPlayer player controls to use the default Material Design vector drawable icons provided by Google.

Here's what the UI looks like with the changes:

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[colinrtwhite]

I signed it!

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CLAs look good, thanks!

[ojw28]

Wont this crash applications running below API level 21 unless (a) they're using the support library and have configured their gradle file appropriately, or (b) they're using Android Studio? I don't think we want to place these kind of limitations on how the library can be used.

Can we add the vector drawables under drawable-v21, but also retain the existing png drawables for backward compatibility?

[colinrtwhite]

The Gradle build process should automatically generate PNG drawables if the user's minSDK is below API 21 using the Vector Asset Studio (no configuration required). However, I agree it makes sense to include PNGs of these vectors in case they aren't using Gradle.

I've updated my PR to include PNGs for devices < API 21.

[ojw28]

Thanks!