Floss: Increase timeout for handle_sigterm

Disconnecting ACL connections with LE devices requires more time.
Increasing the timeout will fix race condition and crash when powering
off the adapter.
Change to 4s because the ACL disconnect timeout can be 2s + 1.5s defined
by BTA_DM_DISABLE_TIMER_MS and BTA_DM_DISABLE_TIMER_RETRIAL_MS.

Bug: 253510389
Bug: 256099903
Tag: #floss
Test: bluetooth_AdapterLEHealth.le_power_toggle_connect_loop.floss
Test: manually power-off with multiple LE devices connected
Change-Id: I7a6d0b1a628f6897d1ea8ae5bdf9f8da33755b5e