Refactor native Zygote code.

This patch refactors the native Zygote code in anticipation of future
changes.  Notibly, this allows forking without application specific
information, replaces a lambda that was defined twice with a function,
and cleans up some of the jstring extraction logic.

Bug: 68253328
Test: Built image, flashed phone, and booted
Change-Id: I694994b2062b2839bf0cdf7d4bef366d98fc6eed