Add the skelton of NativeActivityThread

This CL adds a directory and basic files for building
NativeActivityThread rust library which will be used for managing
native application processes.
This library is built as a dependency of the Native Zygote, which is
guarded by a build flag.

Bug: 402614577
Test: build
Flag: build.RELEASE_NATIVE_FRAMEWORK_PROTOTYPE
Change-Id: I585a356e794054b657ba8f8e3c61c8329f9022bd