Add Hook Options for clang-format

This allows individual projects to enable clang-format pre-upload hook
without worrying about the paths of the various tools.
Bug: 26800693
Test: Added a PREUPLOAD.cfg in external/minijail, got a warning.
Change-Id: Ia9ee738ad3305934e8e6ca50261f96ca3ab30852

GLOBAL-PREUPLOAD.cfg

@@ -4,3 +4,7 @@
 
 [Builtin Hooks]
 commit_msg_test_field = true
+
+[Tool Paths]
+clang-format = ${REPO_ROOT}/prebuilts/clang/host/${BUILD_OS}/clang-stable/bin/clang-format
+git-clang-format = ${REPO_ROOT}/prebuilts/clang/host/${BUILD_OS}/clang-stable/bin/git-clang-format