Mitigate layer leakage in Fullscreen Magnifier

AM#attachAccessibilityOverlayToDisplay() is an IPC. Calling it
multiple times for the same surface is redundant and can increase the
chance of race conditions which causes layer leakage.

This change adds a variable to check the attachment status to avoid
unnecessary attach calls.

Test: Quickly turned on/off the magnifier, the layer leakage is mitigated.
Bug: 407915377
Flag: com.android.window.flags.scvh_surface_control_lifetime_fix

Change-Id: I302c71a5899872b784173b68f899f4d1f98fbc81