manager: return abstract base type

Unit-tests later required knowing the vtable for a BlobManager, and this
should avoid that needless complication.

Change-Id: I7a208d08c2065b7383fd5095e00d17c1ab3b0cf5
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/example/example.cpp b/example/example.cpp
index 9209798..c17d71d 100644
--- a/example/example.cpp
+++ b/example/example.cpp
@@ -174,7 +174,7 @@
 
 void setupExampleHandler()
 {
-    BlobManager* manager = getBlobManager();
+    auto* manager = getBlobManager();
     if (!manager->registerHandler(std::make_unique<ExampleBlobHandler>()))
     {
         log<level::ERR>("Failed to register Example Handler");