Use portal for background and autostart

[ryonakano]

I guess the current way might be blocked on AppCenter Reviews so starting migration to the background portal form now.

This also fixes the Could not acquire service name warning shown on the terminal because we're ditching DBus.

References:

• https://github.com/elementary/mail/commit/37c98e0426c89b2b8f5b0eea8555637faf41f4eb

[ryonakano]

Portals 7.1.0 with Background Portal is now out so merging this branch to master.