Add com.android.mediadrm.signer api files

API txt files for com.android.mediadrm.signer are added.
The contents are empty as they didn't exist before API level 28.

Bug: 77577799
Test: make -j
      mkae checkapi
      make update-api

Change-Id: I91750a55642f93af0322360b6ac89e9c3001c5fb
6 files changed