Better tracking of differences in heap allocations

The previous change to track differences between two allocations
was incorrect - it would change the allocation info itself.

This CL fixes that by actually having a separate object that
stores the differences in allocation count between two snapshots,
but delegating to one of the snapshots for all other information.

Change-Id: I7106a6b2060739db1b3e1d8c048ffc3d082a0d25

• ddms/ddmuilib/src/main/java/com/android/ddmuilib/heap/NativeDiffAllocationInfo.java[Added - diff]
• ddms/ddmuilib/src/main/java/com/android/ddmuilib/heap/NativeHeapDiffSnapshot.java[diff]