Remove usage of AGL_APP_REVISION (AGL_DEFAULT_REVISION)

We did use a variable named 'AGL_APP_REVISION' to switch our apps between the HEAD of the respective branch to the released tag.

While this worked well in the workflows previously, it t urns out that this means bitbake will have to query for these every time it runs (aka cannot trust a tag to be what is local).

For now we'll use fixed SRCREV's like we do for the other sources as well.