Add allocation stack traces for HPROF dump.

This feature is currently only enabled when DDMS's allocation tracking
is enabled. In the future there should be a way to enable this feature
before an application starts.

Also updates DDMS's recent allocation tracking to use a new backend
data structure that is shared with this feature.

The following system properties controls customizable parameters:
dalvik.vm.allocTrackerMax: max number of objects that have allocation
                           records, default 512K;

dalvik.vm.recentAllocMax:  max number of records that are sent to DDMS
                           when clicking "Get allocation" button,
                           default 64K-1 (limit of the protocol);

dalvik.vm.allocStackDepth: max number of stack frames in an allocation
                           record, default 4.

Bug: 20037135
Change-Id: I26ed378a5613678bd3c43e846025f90470a8e059