[flexiglass] Adds corner "ribbon".

Adds a visual indication to the UI that it's running with Flexglass on.
This should prove useful for debugging as it will let developers know
whether they're looking at Flexiglass UI or not.

To see it, the Flexiglass flags must be on and a system property named
flexi.ribbon must be set to true using:

$ adb shell setprop flexi.ribbon true

The ribbon is added as a generic composable such that it can be reused
by other future features, if needed.

Fix: 299340392
Test: manually verified that the ribbon is turned on and off with
Flexiglass feature flags + system property.
Test: see screenshot on the attached bug.

Change-Id: I54e03644fc46b69d7329ae6d3c32c6549ccf34c2