Bluetooth: Mesh: Increase initial poll timeout

The purpose of using something less than the configured poll timeout
was to cover the case where the LPN establishes Friendship before the
provisioner has completely configured it. However, there's the "more
data" flag in the initial Friend Response, and we now also have a
public API to request for more messages. Both of these features
diminish the value of having a reduced initial timeout. Also, some LPN
test cases do not expect us to send frequent polls initially, causing
failures with the PTS.

Therefore, introduce a Kconfig option to set the initial timeout, and
make it default to the actual poll timeout.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>