Robust caching: read database hash on devices with unknown state.

Currently, we would not set the srvc_hdl_db_hash, and would always
trigger full service discovery on such device.
This doesn't match the spec, or the comment just below the if statement.

Bug: 293249869
Test: Bond with a set of devices, where both of them have same GATT
database hash. Verify service discovery is triggered only once in snoop
log.

Test: 1. Connect to device that supports robust caching.
      2. Disconnect.
      3. Connect again to same device, it might roatate it's address in
	 meantime
      4. Verify full GATT database discovery was done only onece, on 2nd
	 connection we read DB hash first.
Change-Id: Icd30ab6157bb411325d64176777ff949203a1985