Create android-base/format.h that includes fmtlib headers

We're already including fmtlib statically in libbase, so let's also
expose its functionality directly.

Also create a small benchmark of the most common operations for
format() and StringPrintf(): int, float, and string arguments.  Note
that fmt::format is faster than StringPrintf() in all of these cases.

Test: build, run benchmark
Change-Id: I1e18f13673dd89ea936ea5c51418fad723495b08