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
diff --git a/28/public/api/com.android.mediadrm.signer-removed.txt b/28/public/api/com.android.mediadrm.signer-removed.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/public/api/com.android.mediadrm.signer-removed.txt
diff --git a/28/public/api/com.android.mediadrm.signer.txt b/28/public/api/com.android.mediadrm.signer.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/public/api/com.android.mediadrm.signer.txt
diff --git a/28/system/api/com.android.mediadrm.signer-removed.txt b/28/system/api/com.android.mediadrm.signer-removed.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/system/api/com.android.mediadrm.signer-removed.txt
diff --git a/28/system/api/com.android.mediadrm.signer.txt b/28/system/api/com.android.mediadrm.signer.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/system/api/com.android.mediadrm.signer.txt
diff --git a/28/test/api/com.android.mediadrm.signer-removed.txt b/28/test/api/com.android.mediadrm.signer-removed.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/test/api/com.android.mediadrm.signer-removed.txt
diff --git a/28/test/api/com.android.mediadrm.signer.txt b/28/test/api/com.android.mediadrm.signer.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/28/test/api/com.android.mediadrm.signer.txt