simpleperf: enable .clang-format

Use the 2-space configuration by default and disable any formatting for
the demo/ subdirectory. The following command was used to generate this
change:

  $ find . \( -name \*.cpp -o -name \*.h \) -exec clang-format \
    --style=file -i {} \;

Test: mm
Change-Id: I89ec1f18f6e352f40cfa1a770087f3b586b1d7cb
103 files changed