clang-format: Double-indent continuation lines

The default value for ContinuationIndentWidth inherited from the Google
style is 4. Since .clang-format-4 doubles the value of IndentWidth
from 2 to 4, it should also double ContinuationIndentWidth to retain
Google's double-indent style.

Before:

 void forEachInterface(
     const std::string& dirname,
     const std::function<void(const std::string& path)) {
     fn(dirname, "default");
     DIR* dir = opendir(dirname.c_str());

After this patch:

 void forEachInterface(
         const std::string& dirname,
         const std::function<void(const std::string& path)) {
     fn(dirname, "default");
     DIR* dir = opendir(dirname.c_str());

Test: applied to libsysutils/src/*.cpp and compared output.
Change-Id: I75c50fa9becd5291a17047933f443cc6db77838a