KeysController: return correct number of unsigned prekeys

When GET /v2/keys was orignally added in b263f47, prekeys were stored in
Postgres, with a user's unsigned and signed keys together in the same table.
Therefore GET /v2/keys subtracted one from the count returned by storage.

In d4d9403, we changed to a different storage schema, with unsigned prekeys in
one DynamoDB table and unsigned prekeys in the accounts Dynamo table.
Unfortunately, GET /v2/keys was not changed to stop subtracting one from the
count of prekeys in the keys table at the same time. This commit fixes that.

service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/KeysControllerTest.java

@@ -199,7 +199,7 @@ class KeysControllerTest {
                                           AuthHelper.getAuthHeader(AuthHelper.VALID_UUID, AuthHelper.VALID_PASSWORD))
                                   .get(PreKeyCount.class);
 
-    assertThat(result.getCount()).isEqualTo(4);
+    assertThat(result.getCount()).isEqualTo(5);
 
     verify(KEYS).getCount(AuthHelper.VALID_UUID, 1);
   }