[APM] New platform migration: replace injected vars

[ogupte]

We rely on injected var in a number of places:

• apmUiEnabled - in public/hacks/toggle_app_link_in_nav.ts, public/register_feature.js
• apmIndexPatternTitle - in public/components/app/ServiceDetails/ServiceIntegrations/WatcherFlyout.tsx
• apmServiceMapEnabled in public/components/app/ServiceDetails/ServiceDetailTabs.tsx, public/components/app/Main/route_config/index.tsx, public/components/app/Home/index.tsx

Related Platform Issue: #41990

Follow the details outlined in the platform PR #50641 to define a set of whitelisted configs that can be referenced from the client.

[elasticmachine]

Pinging @elastic/apm-ui

[smith]

There's some additions in #46497. Search for apmServiceMapEnabled.