Cleanup IME screenshot references

Some IME screenshot methods were referring to removing the IME surface,
when in reality they were removing the IME screenshot.

This also renamed the ImeVisibilityStateComputer#onInteractiveChanged to
shouldShowImeScreenshot to better reflect the actual usage, rather than
the reason for calling it. Lastly, this decouples the IME screenshot
from applyImeVisibility into applyImeScreenshotVisibility.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest DefaultImeVisibilityApplierTest ImeVisibilityStateComputerTest
  DisplayContentTests
  ZOrderingTests
  Showing the IME screenshot (switching apps, turning the screen off and
  then on)
Change-Id: I01ff035db1cddcd3158dc90d6b04f07b32e75313