Add interstitial for modes that are disabled but not by user

This covers both the case of navigating to the mode page through the mode aggregator menu as well as if an app sends an intent to go to the mode.

The interstitial visuals are not done yet; in particular, the image is just a gray box for now.

Manual tests done:
- visually verifying the interstitial page in both portrait and landscape
- accessing the mode page from modes list, confirming interstitial pops up
- accessing enabled and disabled-by-user mode page from modes list (no interstitial)
- getting to the mode page from intent through an app
- accessing enabled and disabled-by-user mode pages from app intent (no interstitial)
- adjusted display and font size (it looks bad with max display & font size, maybe not much to be done)

Bug: 332730534
Test: manual, ZenModeFragmentTest, SetupInterstitialActivityTest
Flag: android.app.modes_ui
Change-Id: I21f13b0842d5b118a341f7d85e8fcac947ca3a06