ANDROID: GKI: Enable CONFIG_KUNIT

Enable the KUNIT core, which has a small ABI impact, and also the
DEBUGFS feature so that results can be reaped. This change does not
actually enable any KUNIT tests or the internal selftests, those must be
enabled by downstream builds.

Bug: 176228452
Change-Id: I2817cb1495fe7bf0485e63f877a68b1971e26cc5
Signed-off-by: Alistair Delva <adelva@google.com>