Consider windows with alpha=0 as invisible

Previously Winscope considered only the `isVisible` property to
determine if the window was visible or not. This failed in cases such as
`com.google.android.googlequicksearchbox/android.app.ContextImpl` when
launching apps, where `isVisible=true` and `alpha=0`

Bug: 181630910
Bug: 182720234
Test: atest FlickerTests
Change-Id: I73de39557f0a9cebaad66d169f81794779793f64
2 files changed