netd: Enable the clang_format preupload hook

Copied verbatim from system/core/PREUPLOAD.cfg.
For more details, see //tools/repohooks/README.md.

Demo usage:

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codewiz@xyzzy:~/src/aosp/system/netd% repo upload .
[COMMIT 15bb55c1] netd: Enable clang-tidy and fix all warnings
[FAILED] clang_format
    The following files have formatting errors:
    	server/SockDiag.cpp
    	server/SockDiag.h
    You can run `[...snip...]` to fix this

An automatic fix can be attempted for the "clang_format" hook. Do you want to run it? (Yes/no)? y
Fix successfully applied. Amend the current commit before attempting to upload again.
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In case clang-format produces unwanted reformatting, the preupload check
can be bypassed by:

  repo upload --no-verify

Test: as shown in demo above
Change-Id: I04f9fc860b894609d493b688f58a04fff362d1d2