ui: add a Field Flamegraph to the Heap Dump Explorer

The heap dump flamegraph is a class tree: it shows which classes retain
memory but not through which fields. Add a Field Flamegraph tab which
puts the field holding each object between its parent and its class
(SystemServer -> .mPackageManagerService -> PackageManagerService -> ..),
for the shortest path and dominator trees.

The trees come from new stdlib modules, android.memory.heap_graph.
field_tree and dominator_field_tree, which have the same totals as the
class trees. Runs of a class held by the same field (e.g. linked lists)
collapse into one level. An object whose dominator does not reference it
gets a '[via Owner.field]' frame naming the field which does.
"Show objects" on a field frame lists the objects the field holds.