Fix link-type check warning on com.android.nfc_extras

The library has been built without SDK, and is used by an app
NfcExtrasTests that is built with SDK. Such this SDK -> non-SDK
dependency has been causing link-type check warnings, which will turn
into errors soon.

This change fixes the warning by making a stub library
com.android.nfc_extras.stubs from the runtime library and let the app to
link against the stub library. Since the stubs library does not use any
private APIs, it is built with SDK.

Bug: 69899800
Test: m -j NfcExtrasTests is successful and does not show any link-type
check warning.

Change-Id: I57980ccbc9036d7cc6df114a975a384d10a2962b