Include Brillo's cannonical .clang-format file.

Many Brillo repositories need to share a common .clang-format file to
ensure the same formatting rules. This patch moves the .clang-format
file already in system/core/metricsd to a common location to be shared
by other Brillo repositories.

Bug: 27121653
TEST=symlink from system/update_engine, ran clang-format.

Change-Id: Ie04a5a9cf54b9cc24f180fe3896501db4d883a64