ndk_library: limit exports

The NDK should stand alone, and the libbinder_ndk case
is created because libbinder_ndk in the NDK used to
contain extra C++ headers, but these were moved into
the SDK to be next to AIDL. Since many modules depend
on these headers, exports are only allowed for this case.

Bug: 357711733
Test: build with neverallow breaking, for instance:

error: frameworks/wilhelm/Android.bp:56:1: module "libOpenSLES.ndk" variant "android_x86_64_silvermont_sdk_shared_21": violates neverallow requirements. Not allowed:
        module types: ["ndk_library"]
        properties matching: "Export_header_libs" matches: .is-set
        EXCEPT in dirs: ["frameworks/native/libs/binder/ndk/"]

Change-Id: I9a32d3cb7f644fb5dbc1e8072894f2f585b2cd6d