Update git submodules

* Update platform/system/tools/aidl from branch 'master'
  to d875b4904bbe094728404ee2c9ed512eabcbcaed
  - Union has its associated (nested) tag enum
    
    For each union, a `Tag` enum type is auto-generated.
    
      union U {
       int a;
       String b;
      }
    
    is equivalent to
    
      union U {
        int a;
        String b;
        @Backing(type="int") enum Tag { a, b }
      }
    
    Bug: 218912230
    Test: aidl_integration_test
    Change-Id: Ic0cb8f4ad9a80fe9a301a97d4f061457fc99756a