IDLE keepalives are not sent frequently enough on Android when the app is suspended

Delta Chat attempts to interrupt IDLE every 5 minutes:
https://github.com/deltachat/deltachat-core-rust/blob/81e9628ab79941adaa12de1e30cefb8d8d69b3f9/src/imap/idle.rs#L14-L20

However, this does not seem to happen because monotonic clock used almost everywhere does not advance when the app is suspended and boottime clocks which should be used instead are not available in Rust yet:
- https://github.com/tokio-rs/tokio/issues/3185
- https://github.com/rust-lang/rust/issues/71860
- https://github.com/rust-lang/rust/pull/132331

FairEmail apparently does not have such problems, but it is using Java APIs everywhere:
https://github.com/deltachat/deltachat-core-rust/issues/6532#issuecomment-2650490127

The bug can be worked around by reducing `imap_idle_notify_interval` in Dovecot, but we need a fix on the client side:
https://github.com/deltachat/deltachat-core-rust/issues/6532#issuecomment-2650490213