SDK / exoplayer

1. When we try and load something via reflection and find the
   class, always throw rather than failing silently if we
   subsequently fail to instantiate an instance. This is
   indicative of a broken proguard setup, and failing silently
   makes it hard to spot.
2. Add library/core proguard configuration to ensure extension
   renderer constructors that we access via reflection are kept.
3. Add demos/main proguard configuration to ensure ImaAdsLoader
   constructor that we access via reflection is kept.
4. Added IMA proguard file to hopefully fix #3723, although I
   wasn't actually able to reproduce the issue.

Issue: #3723

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183648187