(refactor) Updating the field name mIsFullscreenMagnificationActivated

Updating the field name and method names since it is used for indicating the activation of the fullscreen magnification instead of the border status. Therefore, we won't mix up border drawing process with fullscreen magnification activation.

Bug: 291891390
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification
Flag: N/A
Change-Id: I378764c633d2cb97e248b0f5eaad4f4e27cd4caf