Add a 2 width option of clang format.

Make the default the 4 tab space version.

Add a link to the 2 space version for libbacktrace.

The only other difference between the 4 space and 2 space tab version
is that the 2 space tab version allows short functions on a single line.
This is for things like constructors, short destructors, or accessor
functions. I believe this is closer to the Google C++ style guide.

Bug: 36046320

Test: NA
Change-Id: Ida08db18902eeb101f42869dd5590182529d54ef