wifi: Fix SoftAp bridged mode idle shutdown timer

The shutdown timer is shared between two bridged AP instances.
It causes the timer doesn't reset if client connect/disconnect before
the timer expired.

Separate timer for each instance and maintain it separately.

Bug: 204097824
Test: atest -c FrameworksWifiTests
Test: Manual Test, test on single AP mode and bridged Ap mode to check
timer works normally.

Change-Id: I53632260cfc512471ab415a0c407d6fe692310df
2 files changed