Add optimization for LOW_4G allocator

The optimization uses the maps_ field to skip over regions which we
know wont be large enough for the request.

Reduces time to create heap from 500-700ms to 100ms on N9.

(cherry picked from commit 1c8bef4a71612f10b89b102434f70de5a127cc74)

Bug: 20727525

Change-Id: I4fa92d58c2e135ba31a03ababff46669089bb542
1 file changed