system-clang-format: Add Standard: Cpp11

clang-format tries to automatically detect the Cpp standard used,
particularly when determining to put spaces in template definitions:

Cpp03: std::vector<std::pair<bool, bool> >
vs
Cpp11: std::vector<std::pair<bool, bool>>

It doesn't always get this correct as seen in
https://android-review.googlesource.com/c/platform/system/core/+/973463/8
but since we know that all code within Android is C++17, we can safely
assume it should be formatted with the newer standard.

Test: clang-format doesn't break the above change.
Change-Id: I7d1d709690b7bca6da3863cc9a58e53eaec751b0