Refactor ZHA Entity availability tracking (#36645)

* Refactor ZHA entity availability

ZHA entity availability tracks on underlying ZHA device availability.

* Update device status without signal.

* Update tests.

* Fix tests.

* Tests for restored devices availability.

* Guard against empty last_seen entry

Refactor device loading a bit.

tests/components/zha/common.py

@@ -175,10 +175,10 @@ def async_find_group_entity_id(hass, domain, group):
     return None
 
 
-async def async_enable_traffic(hass, zha_devices):
+async def async_enable_traffic(hass, zha_devices, enabled=True):
     """Allow traffic to flow through the gateway and the zha device."""
     for zha_device in zha_devices:
-        zha_device.update_available(True)
+        zha_device.update_available(enabled)
     await hass.async_block_till_done()