I have the same question as you. To answer yours (and mine) – the only guideline I found is the Google Page about Selection. Basically it says that long click on the list item “selects” the item – and moves the Activity into Contextual Activity Bar mode – where activity bar changes to reflect some editing possiblities to the “selected” items.
It is up to you what “edit” of the “selected” items mean. One obvious edit is the ability to delete – hence you see the trash can. Further if this one item can be edited with a separate activity – then you’ll see a “pen” icon. If you select more than one item this “pen” icon will typically disappear.
To me it seems like too many clicks. So I am in favor of the long click to bring up the activity that edits the item. In the example you provided it what the Tasks app does.
One improvement I try to come up with is to make this activity look more like “contextual” editing. One way of doing would be only partially obscuring the original list – so it is not a normal activity but a dialog of sorts.
I did not do that in my app, though. What I did was my second option – namely style the ActionBar to look like CAB (contextual action bar). Notice that Tasks example has the logo, the back sign, and the accept and trash icons are on the left. Typically CAB has check on the right, with a different background, no “back” icon, no “logo” icon and other editing actions are on the left.
Technically it is not trivial to do. One advice I was given on StackOverflow is to force editing activity into ActionMode. That has drawbacks, too long to go into it here. That is the option I took though. Another possiblity is completely custom draw the ActionBar, which I assessed it too much trouble to do now.
That nicety aside, I’d go with the Task option in your case