Fix link-type check warning on com.android.mediadrm.signer

The library has been built without SDK, and is used by an app
GtsCastV2SignerApp 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.mediadrm.signer.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 GtsCastV2SignerApp is successful and does not show any
link-type check warning.

Change-Id: I8a95cc3b14eb2fc4ca90b66aee002f77099187c4