Bluetooth: Mesh: Fix unable send mesh message when proxy adv pending

When Proxy advertising or PB-GATT Advertising Enabled and use a same
advertising sets.

As `adv_start` will call multi HCI Command will cause syswork_q yield.
At same time, if another thread(BT RX) all `schedule_send` will cause
unable send mesh message, because `ADV_FLAG_ACTIVE` was be set, but
`ADV_FLAG_PROXY` not set currentlly.

Add `ADV_FLAG_SCHEDULE_PENDING` indicate mesh buf has been pendings
but not scheduled, so when proxy advertising enable, let's take again,
as we can't break or terminated `adv_start`, so we must waiting proxy
advertising enabled.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>