build: always define NDEBUG in the static analyzer

ART enforces some invariants only in debug builds, denoted by DCHECKs,
"if (kIsDebugBuild)" and the like. To always expose these invariants to
the static analyzer and not trigger warnings for the release build,
always compile in debug mode, that is, ensure that NDEBUG is undefined.

Bug: 32619234
Test: test-art-host. mma showed no false-positive that this was meant
to silence.
Change-Id: I24906b048b9343966d60755cf92be7d670878d81