Ignore to show splash screen if the decorView is not opaque

When the user chooses dark theme, UiModeManager will clear the cache of
task snapshot for this configuration change, which means the system will
reset to use splash screen as the starting window when launching the
app.

However, when the user tries to bring the app to the top, some apps like
Twitter MainActivity uses transparent drawable as the splash window background,
which looks weird that only see the blanked status bar with transparent background
when bring the app task back to top.

To fix that, add a check in PhoneWindowManager#addSplashScreen, when the
view background is not opaque, will ignore this splash screen creation.

Fix: 146080233
Test: manual as below step:
     1. Open Twitter, swipe home.
     2. Quick settings => turn dark theme on. Close shade.
     3. Launch Twitter from home screen
     4. Expect no blanked status bar with transparent background during
        launching.

Change-Id: I9d9713af7285c8c63b86faa90b72690471a202ea