Disable safe region letterboxing for activity embedding activities

- Do not apply a safe region letterbox if activity uses activity
  embedding.
- Remove this once activity embedding support letterboxing.

Bug: 404940558
Test: atest CtsWindowManagerJetpackTestCases
Flag: EXEMPT (bug fix)
Change-Id: Ia558d9b58dfd55eb655d5d2ba7cf3e2ce69d8da4