Update apidiff.mk to use the sdk-name macro.

This internal change complements aosp/661901, which introduced the sdk
module name macro. apidiff.mk doesn't exist in aosp.

Bug: 77525052
Test: make droid
Change-Id: Ia347d8ffef1a7d28508256b4a5c984b3e07ed998
(cherry picked from commit d4e657a2)