Fix the way malloc debug returns info.

When I rewrote malloc debug, I accidentally thought that each
value returned in the info buffer contained the number of backtrace
frames in the backtrace. This was incorrect, it should have been
the total number of allocations with the same backtrace.

This is a temporary fix that sets that value to 1. The better fix is
to de-dupe backtraces and then return all allocations of the same size
with the same backtrace.

I updated the documents to describe this.

Bug: 31854476

Test: Unit tests pass.
Change-Id: Idf9efaa3d363923b5d7543d90dc7c65a0ed553d9