Bug/certificate expiration #5821

This makes bitmask android to download a new certificate if the current one is going to expire within 7 days.

To test this, I've used this code on EIP.java

As a global variable:     private static int simulate_certificate_expiration = 0;
To artificially make the certificate "obsolete", in private void checkCertValidity():
	Log.d(TAG, "simulate certificate expiration = " + simulate_certificate_expiration);
	if(simulate_certificate_expiration > 4) {
	    offset_date.add(Calendar.DATE, 120);
	    simulate_certificate_expiration = 0;
	} else 
	    offset_date.add(Calendar.DATE, 7);
	simulate_certificate_expiration++;