logd: use libbase logging

We can use libbase logging to output to the kernel log instead of the
'prdebug' function, so use that instead.

Bonus #1: we can now use CHECK().
Bonus #2: logging unit tests automatically output to stderr.
Bonus #3: We see dependent library's logs instead of losing them to
the void.

Test: logging unit tests
Test: logs show appropriately in dmesg / stderr
Test: CHECK() works
Change-Id: I92f8056b4820dc4998996cf46460568085299700