Optimize code that only uses PageMap call.

Some code only wants to use PageMap to determine whether pages are
mapped in to memory. Modify the code to optimize this path.

Changes:
- Add a function that doesn't read all of usage stats data for every
  map. This operation is extremely expensive, and some code doesn't
  care about it.
- Optimize the PageMap call to do a single read instead of a single
  read per page.
- Add unit tests for these changes.

Bug: 136245508

Test: Ran new unit tests.
Test: Ran procrank and verified data looks good.
Test: Ran DexDiag art tests.
Change-Id: I37d03f2584551d26cb20be3abacdb958111d4eca