Change test storage to use protobuf

[ Merge of http://go/wvgerrit/193190 ]

This changes the persistent test storage to use protobufs instead of
manual parsing.  This simplifies the code but makes the files less
"human readable".  Files can be read using 'gqui' if needed.

Bug: 312529037
Test: unit/integration tests
Change-Id: I1b025eac96458c0061e0883e1e4fd05484842ff2

libwvdrmengine/cdm/core/src/device_files.proto

@@ -20,6 +20,10 @@ message NameValue {
   optional string value = 2;
 }
 
+message SavedStorage {
+  map<string, string> files = 1;
+}
+
 message OemCertificate {
   enum PrivateKeyType {
     RSA = 0;