Update git submodules

* Update prebuilts/androidx/external from branch 'androidx-master-dev'
  to 0f4a7eb7517ae98ecc6a8b4693b238a4a9fa3a44
  - Merge "Update metalava to v1.0.0-alpha3" into androidx-master-dev
  - Update metalava to v1.0.0-alpha3
    
    ./development/importMaven/import_maven_artifacts.py -n   com.android.tools.metalava:metalava:1.0.0-alpha03 -mb 7027994
    
    Bug: 171340471
    Bug: 171360120
    Test: None
    Change-Id: Ib6db2e526d54baf52e0d37ce52619cd1a7e0bfc7