Add signature|system permission to MediaDrm signer APIs

Update test to require the system permission

Merge of https://widevine-internal-review.googlesource.com/#/c/9652
from the Widevine cdm repo.

Change-Id: I071e55f09f85c420e6778ebf170f5110cf39809e
related-to-bug: 12702350

libwvdrmengine/test/castv2/AndroidManifest.xml

@@ -4,6 +4,7 @@
 >
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
+    <uses-permission android:name="android.permission.ACCESS_DRM_CERTIFICATES" />
     <uses-library android:name="com.android.mediadrm.signer"
                   android:required="true" />