drmserver: add readproc group

drmserver need to be in the readproc group to access /proc files
associated with other UIDs, due to enabling hidepid=2 in
https://android-review.googlesource.com/181345

Bug: 25925909
Change-Id: If477214a6cd598cebbe440a94aa1d58ea8f7114a
diff --git a/drm/drmserver/drmserver.rc b/drm/drmserver/drmserver.rc
index c6b8226..42f5fc8 100644
--- a/drm/drmserver/drmserver.rc
+++ b/drm/drmserver/drmserver.rc
@@ -1,4 +1,4 @@
 service drm /system/bin/drmserver
     class main
     user drm
-    group drm system inet drmrpc
+    group drm system inet drmrpc readproc