libbinder_ndk: separate libbinder plat headers

binder_ibinder_platform.h is a better name for platform extensions, and
we don't need specific headers like binder_context.h. Fixing the
motivating problem for splitting these (that binder_ibinder_platform.h
contains references to libbinder C++) by putting that in its own file.
This is cleaner because we expect many platform extensions to
libbinder_ndk, to support Rust, or to avoid using libbinder, so the
conversion functions can be split off instead.

Bug: N/A
Test: build
Change-Id: Idb00abd10326b3d7346e8e2f99e2186b32513af7