Reset the alpha value of ALL notification content views

* After a hideSensitive change
* After the appear animation
* After an activity launch expand animation

This is intended to solve a bug where the notification would appear as a persistently blank rectangle.  From bugreports, it seems clear that the content view of the notification is having its alpha left at 0 (or something low in at least 1 case).  The best explanation for this bug at the moment is that one of these 3 animations (most likely either the appear animation) has a race condition whereby the content view which is set to 0 or less for the animation is no longer the active content view when the animation continues/ends, resulting in the alpha being left with a value other than 1.  When another change results in this content view showing again, that change assumes the content view has the correct alpha, so the alpha is left incorrect.

Bug: 188770583
Test: manual testing of all 3 animations listed above
Change-Id: I9e9aeb5dd3643bde1dc9f2ffe8b32d7d993b29f2