Add a new module-only API to preload HttpEngine

This is supposed to be used only from the Zygote during the
initialization phase to preload HttpEngine. Currently, the
implementation only preloads a single class which is used to initialize
the backend for HttpEngine.

Bug: 380349437
Change-Id: If3ebcb37e51104a69473491e9f230d1b043c7bd0
2 files changed