Introduce ZygoteInit.preForkInit().

Until recently, I and apparently some others were under the assumption
that RuntimeInit.commonInit() runs before the Zygote fork. Actually, it
does not.

This CL introduces a method ZygoteInit.preForkInit() which runs before
the Zygote fork. For now, only enableDdms() moves into it and can become
private. This should not alter behavior since enableDdms() is called
from the same places as before, and because enableDdms() (now private)
was already not part of any API surface.

The CL also removes the qualifier "final" from the (static) method
enableDdms(), because it is redundant.

Note: This CL was uploaded with --no-verify because of preexisting
import ordering issues in RuntimeInit.java

Test: Treehugger
Change-Id: I8f637e160a2d7810feb43b6a43ec7d628af18fb8