Fixes https://github.com/flutter/devtools/issues/8155
Previously after enabling Widget Selection mode from DevTools and
selecting a widget to inspect, a user would then have to click the
on-device "Select widget" button before being able to select another
widget. This was very confusing to users; we got multiple comments on
our latest DevTools Survey that widget selection mode only worked the
first time and was broken on subsequent selections.
Now, once "Select widget mode" is enabled from DevTools, any subsequent
click is treated as a selection until the user exits from select widget
mode either via DevTools or via the Exit Selection mode button.
The user can re-position the Exit Selection button to either the left or
the right of their device (this way they can select a widget beneath
it).
Note: Previously this button was behind any widget selection overlays.
This PR also updates the order of the `Stack` so that exit selection
button is on top.
a051be8585