Don't allow negative GetFreeMemory.

The max allowed footprint is only updated after the GC. But we can
still allocate even if bytes_allocated > max_allowed_footprint_.
This means that we used to be able to get a negative value if
bytes_allocated > max_allowed_footprint_.

External bug:
https://code.google.com/p/android/issues/detail?id=72221

Change-Id: I4ef9a534e29211786e82cdcb2582c11ab37a348a
1 file changed