bug(mat-sidenav): focus trapping is too aggressive under certain conditions

[mmalerba]

Reproduction

This issue seems to manifest under the following conditions:

• Sidenav is open in over or push mode (does not happen in side mode).
• Backdrop is disabled (this is necessary to be able to click in the content area of a over or push sidenav.
• App is using the ConfigurableFocusTrap (does not happen with the deprecated FocusTrap).

Steps to reproduce:
https://stackblitz.com/edit/angular-avy29p?file=src%2Fmain.ts

1. click to focus the "Content" input

Expected Behavior

"Content" input remains focused.

Actual Behavior

"Drawer" input immediately steals focus.