Update git submodules

* Update frameworks/support from branch 'androidx-main'
  to 4480c0687f762606269ac9bafc433d1bcf6c2974
  - Merge "Upgrade to robolectric 4.7.3" into androidx-main
  - Upgrade to robolectric 4.7.3
    
    Test: ./gradlew test
    Change-Id: I464210120ff528b0e760f580803515ec5f068aaa
    

* Update prebuilts/androidx/external from branch 'androidx-main'
  to afc762544addd58683ec0a8c9201a24d0ed39a89
  - Merge "Add prebuilts for robolectric 4.7.3" into androidx-main
  - Add prebuilts for robolectric 4.7.3
    
    Test: ./gradlew test
    Change-Id: I42782330dd70916e1ece0c6f2c8d0e346626af8d