Add NDK headers for native services

This CL adds header files which define APIs used for developing native
services.
The headers are placed to a private directory and native_service.h will
be made public when the feature is approved.

Bug: 402614577
Test: build
Flag: EXEMPT NDK headers
Change-Id: I0c244d6a8698cb1022e28d30f0edf0ce99955683