Send subtree changed AccessibilityEvent for all alpha changes.

Changes to view.mtransformationInfo.mAlpha outside of View.java weren't triggering subtree AccessibilityEvents.
This is a problem, because that property helps determine a views visibility in accessibility, and AccessibilityServices wouldn't be informed of this change in visibility.
This results in UIs with certain animations not being accessible at all.

This seems like an oversight to begin with, so it's a pretty low risk change.

Test: Tested the reported instance of this bug in Permission Manager, CTSAccessibility*
Change-Id: I02526e5659cf95f1373811008e74954a73addd21
Fix: 127589394