Improve navigation drawer UX

When a user selects a folder from the drawer we first close the drawer and wait for the animation to be done. Only then do we perform the fragment transaction to switch out the message list. This was done to get a smooth closing animation that isn't interrupted by the new fragment being loaded. But it makes the UI feel slow.

Investigate what exactly is causing the animation stutters when loading the fragment simultaneously. If we can fix it, do that.
Otherwise investigate which of these options leads to a smoother UX:

• When a folder is selected show a blank area where the message list goes, then play the closing animation, then load the new fragment (Gmail seems to do something similar).
• When a folder is selected, keep the drawer open and load the new fragment. Once the new message list has been loaded close the drawer.

Start by reproducing folder selection in the navigation drawer and profiling the drawer-closing animation while the message-list fragment loads. Compare the reported blank-area and load-before-close alternatives, and determine which behavior avoids visible stutters. Done means the selected approach provides a measurably smoother transition without interrupting navigation.