Fix google calendar disabled entity handling

[allenporter]

Proposed change

Fix google calendar disabled entity handling by waiting to poll the entity until it has been added to home assistant, rather than before it is added.

I noticed that Google Calendar was taking a long time to start when loading my corporate calendar which is massive. I first attempted to make all the coordinator loads happen in parallel which helped. Then I disabled the entity since i'm not using it and noticed it still was being loaded. I realized the right fix is to defer the fetching until adding the entity so that the disabled bit is respected, rather than just making the existing calls happen in parallel. This copies what the flume integration does.

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[jbouwh]

Looks good, thnx