Update git submodules

* Update platform/system/update_engine from branch 'master'
  to 596a3206dac75841719f76e67f64576cfaca2331
  - Add an faster OTA extractor
    
    delta_generator is already capable of extracting images from an OTA.
    However, it was created for testing purposes. Code in delta_generator
    tries to mimic device environment as much as possible, and therefore
    does not have optimal performance. Now go/bts-image-sharing project
    needs an ota extractor in the cloud, re-write a faster alternative.
    
    Test: ota_extractor ota.zip /tmp/extracted, 364s -> 272s, 25% faster.
    
    Bug: 203712723
    Bug: 221875271
    Change-Id: Ie30375f2cd1df1890fc58a71f96b3aa430f0fd75