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/native-activity
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/networking
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/tracing
Bug: http://b/77236573
Change-Id: Ic3154f77de1f13fe753b4c03537eab41b0a22848