Add analytics events for background image upload

[jaclync]

trafficstars

We added 6 new events for tracking background image upload feature in iOS https://github.com/woocommerce/woocommerce-ios/pull/7196, please feel free to skip any events that don't apply to Android since our UX is slightly different.

event name	trigger	props
*_saving_product_after_background_image_upload_failed	When the product cannot be saved after none of the images are pending upload in the background.	error, type:product|variation
*_saving_product_after_background_image_upload_success	When the product or variation is saved after none of the images are pending upload in the background.	type:product|variation
*_failure_saving_product_after_image_upload_notice_shown	A notice is shown from failure saving product after image upload in the background.	type:product|variation
*_failure_saving_product_after_image_upload_notice_tapped	The user taps on a notice to view product details from failure saving product after image upload in the background.	type:product|variation
*_failure_uploading_image_notice_shown	A notice is shown from failure uploading an image in the background.	type:product|variation
*_failure_uploading_image_notice_tapped	The user taps on a notice to view product details from failure uploading an image in the background.	type:product|variation

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