Reduce extra a11y events when alpha changes.

Changes in alpha only matter if they affect visibility,
so only 0 <-> nonzero changes are worth reporting. Report
them as subtree changes, as visibility affects subviews.

Not reporting every change greatly reduces the number of
event reported when alpha is animated.

Bug: 30183085
Change-Id: I905d53aa81ca8248b3aed86a91842ef499f303a8