Fix subscribe links for Developer Forums doc.

The email-based subscribe links for the Android Developer forums do
not work for two groups (android-discuss, android-beginners) due to
recent changes in those groups. These links need to be changed to
HTML-based subscribe links instead.

For consistency, other links are being changed to use the HTML-based
subscription mechanism as well.

Internal Bug: 2908002

Change-Id: I601b959935584270d6107c4600b28ac5f51359e1