Remove findClass and mClasses from AhatSnapshot.

The mClasses data structure mapping class names to class objects was
only used for testing purposes. Remove this data structure from
AhatSnapshot and find an alternative approach for the test cases so
that users don't incur the runtime and memory costs of the data
structure.

Test: m ahat-test

Change-Id: If7804f943c6155448a5fded92ff71e946435b584