Add missing @addtogroup tags.

These NDK docs weren't in any groups, so they don't show up in the
new site.

Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/logging
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/sync
Bug: http://b/77236573
Change-Id: I300f96585dec94359ecf96fbf39c6dc3ec4579e5