Sort out ANDROID_GOALS, KATI_GOALS and NINJA_GOALS.

ANDROID_GOALS: any Android goals that need to be built.
KATI_GOALS: goals that we need to pass to Kati.
NINJA_GOALS: goals we need to pass to Ninja.

For modifier Android goals (dist, INTERNAL_MODIFIER_TARGETS),
we don't need to pass them to Ninja. See also commit
80e46c7c.
Restore the dist rule removed by the above commit.

Previously "droid" was never passed to Ninja. That's incorrect.
"make droid docs" should build both docs and droiod.
Fixed with this change.

Change-Id: I5c28061fe0ebe7848872ee349056d029b787ff71