Don't show 'Upgrade available' for agents in hosted policies (#97517)

## Summary
Close item 3 on #91906. Don't show icon & "upgrade available" for agents in hosted agent policies

### Local testing
I'll try to come back and post the exact commands if anyone wants them, but my rough flow was

1. start ES in 7.13
2. checkout kibana 7.x branch and start
3. create agents in kibana
4. stop kibana & ES
5. start ES in 8.0.0-SNAPSHOT
6. switch to kibana `master` branch & start
7. Navigate to http://localhost:5601/app/fleet#/fleet/agents and observe agents with upgrade available
8. assign agent(s) to a hosted agent policy
9. Observe behavior shown in two screenshots
### Don't show "upgrade available" for hosted agents in list
<img width="947" alt="Screen Shot 2021-04-19 at 3 08 12 PM" src="https://user-images.githubusercontent.com/57655/115291869-421c2480-a123-11eb-9dd2-6e450775c16b.png">

### Don't show agent in list when "Upgrade available" filter is selected
<img width="945" alt="Screen Shot 2021-04-19 at 3 08 40 PM" src="https://user-images.githubusercontent.com/57655/115291870-421c2480-a123-11eb-9309-09e3de8902f8.png">


closes #91906

x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_list_page/index.tsx

@@ -407,7 +407,7 @@ export const AgentListPage: React.FunctionComponent<{}> = () => {
           <EuiFlexItem grow={false} className="eui-textNoWrap">
             {safeMetadata(version)}
           </EuiFlexItem>
-          {isAgentUpgradeable(agent, kibanaVersion) ? (
+          {isAgentSelectable(agent) && isAgentUpgradeable(agent, kibanaVersion) ? (
             <EuiFlexItem grow={false}>
               <EuiText color="subdued" size="xs" className="eui-textNoWrap">
                 <EuiIcon size="m" type="alert" color="warning" />
@@ -593,7 +593,15 @@ export const AgentListPage: React.FunctionComponent<{}> = () => {
             emptyPrompt
           )
         }
-        items={totalAgents ? agents : []}
+        items={
+          totalAgents
+            ? showUpgradeable
+              ? agents.filter(
+                  (agent) => isAgentSelectable(agent) && isAgentUpgradeable(agent, kibanaVersion)
+                )
+              : agents
+            : []
+        }
         itemId="id"
         columns={columns}
         pagination={{