Update the "Android Performance" documentation.

A lot of this documentation isn't even true of the G1, let alone Froyo running
on a Nexus One. Distinguish between truth and fiction, clarify where the JIT
affects things, and clarify certain confusions (such as the difference between
intrinsics and native methods).

I still need to include updated performance numbers in the final section. I
should also make the benchmark code available so that people don't have to
take our word for these things, and so it's easier for them to get an idea of
the performance of future devices and builds. (Though hopefully we can update
this every release in future.)

Anyway, just removing the untruths is a big step forward.