Update git submodules

* Update prebuilts/androidx/external from branch 'androidx-master-dev'
  to d9cb5dab530a6561990af96a55ca0486595bd3cf
  - Merge changes Iec59495d,I1c3584f9 into androidx-master-dev
    
    * changes:
      Remove unused robolectric 4.3 prebuilts
      Remove robolectric 4.3.1 and associated unused prebuilts
    
  - Remove unused robolectric 4.3 prebuilts
    
    Bug: 162018163
    Test: ./gradlew bOS
    Change-Id: Iec59495d93966507742de659433cf4e58df8b0d2
    
  - Remove robolectric 4.3.1 and associated unused prebuilts
    
     This is esentially a revert of commit
     9178948e6fe9f68503917daf5d307b12dd8859f8, but maintains prebuilts that
     are being used for robolectric 4.4-alpha-2
    
    Bug: 162018163
    Test: ./gradlew bOS
    Change-Id: I1c3584f9f9ee3e417f845c1fd888af60343d5264
    
1 file changed