Update git submodules

* Update ndk from branch 'ndk-release-r18'
  to 477efcead24f1107e0200bbf75176913b38e9d01
  - Merge "ndk-which: support all ABIs." into ndk-release-r18
  - ndk-which: support all ABIs.
    
    Also switch the default to armeabi-v7a rather than armeabi which was
    removed in r17.
    
    We should probably just remove this tool, but as long as it's there,
    we should make it work.
    
    Bug: android/ndk#755
    Bug: android/ndk#56
    Test: `ndk-which --abi x86 objdump` and `ndk-which objdump`
    Change-Id: I034ee26d1a695bf771a0f38e0097650b61a78ca6
    (cherry picked from commit e97023726375a8bd49a0e4da376b1da12624c3b3)