I would recommend setting this via a local/viewstates.conf in your app. For instance:
TimeRangePicker_0_1_0.default = Last 7 days
There are 2 variables here that will need modification.
mydashboard should reflect the name of your view
_0_1_0 should reflect the module ID of TimeRangePicker in your view. This can be found in a couple of different ways. The easiest of which is probably by looking at the viewstates.conf from your users directory. $SPLUNK_HOME/etc/users/<your_user>/<your_app>/local/viewstates.conf. You could also specify "?showsource=1" in the browser URL just after the view name. This will give you a breakdown of modules and their respective IDs.