Add method to get usage stats for a single map.

The new scudo allocator creates a huge map in 64 bit address space. Some
tests would get the usage stats of all maps, but only really cared about
a small set of maps. In this case, provide a method to only get the usage
stats for the maps you care about.

Test: Unit tests pass.
Change-Id: Ie845e47a8c789a57bf689c9f0e5878a921640e30