Force null_driver.cpp to use _FORTIFY_SOURCE=2

We plan to make _FORTIFY_SOURCE=3 global on device. The null_driver.cpp
file uses malloc_usable_size and this particular case does not work with
_FORTIFY_SOURCE=3. We redefine it to 2 for only this file.

Flag: EXEMPT bugfix
Bug: 291762537
Test: build with ag/32745870 and presubmit
Change-Id: I8a31ba5f367f2a239f19b0fe33c9bc1f6bc8b97c