title: onBackPressed() not called first time from AppCompatActivity
body: <p>I have a series of fragments that are called in order A > B > C from user interaction. The Navigates by clicking "Launch B" from A, and "Launch C" from B.</p>
  
  <p>If the user is on C, and clicks back, I need to clear an Activity level variable, so I have an interface/callback that is set. When my activity overrides onBackPressed(), the callback is called.</p>
  
  <p>I am then on Fragment B, and I press back again. onBackPressed() is not called this time. Pressing it a second time does call it.</p>
  
  <pre><code>public interface BackOverrideCallback {
      void onBack();
  }
  
  private BackOverrideCallback mBackOverride;
  
  private BackOverrideCallback mFragCBack = new BackOverrideCallback () {
      @Override
      public void onBack() {
          mVar = null;
          getSupportFragmentManager().popBackStack();
          mBackOverride = null;
      }
  };
  
  @Override
  public void onBackPressed() {
      if (mBackOverride != null) {
          mBackOverride.onBack();
      } else {
          super.onBackPressed();
      }
  }
  </code></pre>
  
  <p>My API 19 emulator Logcat shows this when the back press does nothing:</p>
  
  <pre><code>com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xb891dad0: ver 2 0
  system_process E/SoundPool: error loading /system/media/audio/ui/Effect_Tick.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/Effect_Tick.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/Effect_Tick.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/Effect_Tick.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/Effect_Tick.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/KeypressStandard.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/KeypressStandard.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/KeypressSpacebar.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/KeypressSpacebar.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/KeypressDelete.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/KeypressDelete.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/KeypressReturn.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/KeypressReturn.ogg
  system_process E/SoundPool: error loading /system/media/audio/ui/KeypressInvalid.ogg
  system_process W/AudioService: Soundpool could not load file: /system/media/audio/ui/KeypressInvalid.ogg
  system_process W/AudioService: onLoadSoundEffects(), Error -1 while loading samples
  </code></pre>
  
  <p>API 27 Logcat shows this</p>
  
  <pre><code>? W/audio_hw_generic: Not supplying enough data to HAL, expected position 30719740 , only wrote 30719520
  com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xa59843c0: ver 2 0 (tinfo 0xa59835d0)
  system_process D/AutofillManagerService: onBackKeyPressed()
  </code></pre>