Make MemoryDealer::deallocate private

MemoryDealer allocate returns Allocation objects, whose
destructors call MemoryDealer::deallocate. Any direct call to
MemoryDealer::deallocate will lead to a double deallocate.
This method is currently not used anywhere, however leaving it
public could introduce future bugs.
 - We make the method private so only Allocation can call.
 - Remove deallocate from fuzzing test.

Test: Compiles.
Bug: 201562507
Change-Id: I3fc4b1a2c30fe9aebf0e22fa5a4547f9940d1427