wlan: Start idle scan on receiving enable aggregation indication

On receiving disable aggregation indication, device is put
into full_power mode and idle scan timer cancel flag is set
to true. Idle scan timer will not be started on processing
exit_imps command due to the idle scan timer cancel flag.
The timer is not restarted on receiving the enable_aggregation
indication but only the cancel flag is reset to false.

Fix is to start idle scan on receiving enable aggregation
indication.

Change-Id: Ie047d1feff3eb4147579aa67c3e38aefe2d6f230
CRs-Fixed: 2342407