Refactor ahat's perflib api.

This change substantially refactors how ahat accesses heap dump data.
Rather than use the perflib API directly with some additional
information accessed on the side via AhatSnapshot, we introduce an
entirely new API for accessing all the information we need from a heap
dump. Perflib is used when processing the heap dump to populate the
information initially, but afterwards all views and handlers go
through the new com.android.ahat.heapdump API.

The primary motivation for this change is to facilitate adding support
for diffing two heap dumps to ahat. The new API provides flexibility
that will make it easier to form links between objects in different
snapshots and introduce place holder objects to show when there is an
object in another snapshot that has no corresponding object in this
snapshot.

A large number of test cases were added to cover missing cases
discovered in the process of refactoring ahat's perflib API.

The external user-facing UI may have minor cosmetic changes, but
otherwise is unchanged.

Test: m ahat-test, with many new tests added.
Bug: 33770653

Change-Id: I1a6b05ea469ebbbac67d99129dd9faa457b4d17e

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