AndroidConfig.h: ensure HAVE_MALLOC_H always has a value.

Some programs (cough sqlite cough) do an improper check to see if
HAVE_MALLOC_H is defined. Instead of defining it with no value,
define it with a value so those checks pass.

Needed for bug: 20099586

Change-Id: I4c293b2c1b5f4caad85ffc1fc34ac385bab23f5c