Don't allow requesting the assistant role.

By adding a requestable attribute for roles and set it to false for
assistant.

Bug: 130735926
Test: atest RoleManagerTest#requestAssistantRoleThenDeniedAutomatically
Change-Id: I66b4e203f2922645e3a357ca0f628faefadfe344
diff --git a/PermissionController/res/xml/roles.xml b/PermissionController/res/xml/roles.xml
index 617fc4d..36cc47e 100644
--- a/PermissionController/res/xml/roles.xml
+++ b/PermissionController/res/xml/roles.xml
@@ -80,8 +80,7 @@
         exclusive="true"
         showNone="true"
         label="@string/role_assistant_label"
-        requestDescription="@string/role_assistant_request_description"
-        requestTitle="@string/role_assistant_request_title"
+        requestable="false"
         shortLabel="@string/role_assistant_short_label">
         <required-components>
             <!-- Qualified components are determined int AssistantRoleBehavior. This comment here is