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
