Update git submodules

* Update platform/external/libavc from branch 'master'
  to 6ff3e24351046b63f774bc2381a5a2be515a3eec
  - Merge "Builds: Modular cmake scripts added"
  - Builds: Modular cmake scripts added
    
    Cmake scripts are now modular. Separate scripts now handle
    build specifications for each target and dependency.
    
    Toolchain files have been added for armv7, armv8 builds.
    
    Bug: 213579857
    Test: Builds using cmake
    Test: Builds in OSS-fuzz
    
    Change-Id: Iac62b443c0a0de09a2ce6553a660db2c3d5dc846