Update docs for WindowInsets#getPrivacyIndicatorBounds

Added a paragraph to be more descriptivea bout how the bounds are
loaded, and when they would be expected to change. In particular, added
a clause to state that the bounds should never be null so long as there
is a StatusBar window.

Fixes: 189190379
Test: read docs
Change-Id: I0392310d3a19ea6f6a74010266e90ffe5c1543e9