Bluetooth: Mesh: Remove START_PENDING flags

Remove START_PENDING flag, and replace with
bt_mesh_adv_is_empty_by_tag

not only make adv_ext.c more readable, but also enhancement

In previous implementation, after proxy started, will check
this flag, and when the flag is true, will direct stop proxy
and re-enter schedule, but has one problem, this flag set true,
but buf will be empty, such as when sending mesh relay messages
will for-each every sets, until find not-active, but also set
previous set START_PENDING flags, so will cause proxy advertising
started->stop->started.

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