title: Keep class "implements" when Proguard obfuscating
body: <p>My problem is a class definition is changed after Proguard <strong>obfuscation</strong> is run 
  (Mind it is not changed when -dontobfuscate turned on!).</p>
  
  <pre><code>package com.google.ads.mediation.inmobi;
  
  ...
  import com.google.ads.AdSize;
  import com.google.ads.AdRequest.ErrorCode;
  import com.google.ads.mediation.MediationAdRequest;
  import com.google.ads.mediation.MediationBannerAdapter;
  import com.google.ads.mediation.MediationBannerListener;
  import com.google.ads.mediation.MediationInterstitialAdapter;
  import com.google.ads.mediation.MediationInterstitialListener;
  import com.google.ads.mediation.inmobi.InMobiAdapterExtras;
  import com.google.ads.mediation.inmobi.InMobiAdapterServerParameters;
  import com.inmobi.commons.AnimationType;
  import com.inmobi.commons.GenderType;
  import com.inmobi.commons.InMobi;
  import com.inmobi.monetization.IMBanner;
  import com.inmobi.monetization.IMBannerListener;
  import com.inmobi.monetization.IMErrorCode;
  import com.inmobi.monetization.IMIncentivisedListener;
  import com.inmobi.monetization.IMInterstitial;
  import com.inmobi.monetization.IMInterstitialListener;
  import com.inmobi.monetization.IMInterstitial.State;
  ...
  
  public final class InMobiAdapter implements MediationInterstitialAdapter&lt;InMobiAdapterExtras, InMobiAdapterServerParameters&gt;, MediationBannerAdapter&lt;InMobiAdapterExtras, InMobiAdapterServerParameters&gt; {
  </code></pre>
  
  <p>is turned into:</p>
  
  <pre><code>package com.google.ads.mediation.inmobi;
  
  ...
  import com.google.ads.AdSize;
  import com.google.ads.AdRequest.ErrorCode;
  import com.google.ads.mediation.MediationAdRequest;
  import com.google.ads.mediation.MediationBannerListener;
  import com.google.ads.mediation.MediationInterstitialListener;
  import com.google.ads.mediation.inmobi.InMobiAdapterExtras;
  import com.google.ads.mediation.inmobi.InMobiAdapterServerParameters;
  import com.inmobi.commons.AnimationType;
  import com.inmobi.commons.GenderType;
  import com.inmobi.commons.InMobi;
  import com.inmobi.monetization.IMBanner;
  import com.inmobi.monetization.IMBannerListener;
  import com.inmobi.monetization.IMErrorCode;
  import com.inmobi.monetization.IMIncentivisedListener;
  import com.inmobi.monetization.IMInterstitial;
  import com.inmobi.monetization.IMInterstitialListener;
  import com.inmobi.monetization.IMInterstitial.State;
  ...
  
  public final class InMobiAdapter {
  </code></pre>
  
  <p>My proguard settings are below so I that is why the code of InmobiAdapter class is not obfuscated. However my <strong>MediationInterstitialAdapter interface etc are thrown out</strong> from there:</p>
  
  <pre><code>&gt; -optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/*
  &gt; -optimizationpasses 3
  &gt; -allowaccessmodification
  ...
  &gt;  -keep class com.google.** { *; }
  </code></pre>
  
  <p>What should I set to keep that <strong>"implements ..."</strong> part</p>