Revise string and change asset

Change icon to a11y asset and change string based on
String doc

Bug: 174336318
Test: 1. adb shell settings put secure \
      accessibility_show_window_magnification_prompt 1
      2. use full-screen magnification to see if it works well
      atest WindowMagnificationPromptControllerTest
Change-Id: I110ca30b95e268bcad5e9109d7715e854118f0db