Remove functionality related to external allocation tracking.

The android.os.Debug class exposed two interfaces related to external
allocation.  The first interface provided a counter of global external
allocation events.  This change makes the global counter getters
constantly return 0 and stubs out the counter setters.  The second
interface attempted to provide a counter of per-thread external
allocation events.  That interface was never implemented.  This change
makes the per-thread counter getters constantly return -1 and stubs
out the setters.

Change-Id: I240fe9bf05297a83607a4350d40d20581e2a9e2f