Resize & round corners on a placeholder interstitial image

This isn't a real image yet (just draws a diagonal gradient), but should now be roughly ready for inserting the correct drawables based on the mode whenever those are available.

Bug: 332730534
Test: manual, SetupInterstitialActivityTest
Flag: android.app.modes_ui
Change-Id: I45a78950f671a989258a85525a7a8ee0c445a332