Update git submodules

* Update platform/packages/modules/NetworkStack from branch 'master'
  to 52f7eb12c80a4aed9b1e768cf9aa5aa1f26189b7
  - Address comments on shim library changes.
    
    Address comments on aosp/1598153 and aosp/1598155.
    
    Also remove an unused import and fix some javadocs.
    
    Test: treehugger
    Change-Id: I2ec0aa737804cc3ea5fce9326f84c1e4b5e4972c
    
1 file changed