Check for attached state explicitly before triggering data stall recovery

The current implementation only checks for isDataAllowed() which could
return true based on mAutoAttached flag even though GPRS is detached.
This CL adds additional check for mAttached flag to prevent data stall
recovery for such cases.

Test: Manually verified by changing preferred network type
      atest com.android.internal.telephony.dataconnection.DcTrackerTest
Bug: 127425834

Change-Id: I14f3978c37ff02f5016c08efbbb8dec96dde4715