Add a scrim to taskbar for bubbles expanded state

When bubbles are expanded a scrim is shown on top of
everything. Taskbar is layered above bubbles but we still
need the scrim to show on top of it. This CL adds the
ability to show a scrim on taskbar.

The scrim is a view in the taskbar drag layer and is
placed between the taskbar and the nav buttons so that
it can block touches / scrim the taskbar but still allow
the nav buttons to be visible and touchable.

Add interpolators for alpha matching what bubbles is using.

Test: manual 1 - expand bubbles while taskbar is visible
                 => observe scrim
               - open manage menu
                 => observe darker scrim
             2 - check that taps on scrim collapse manage
                 menu or stack
             3 - check there isn't a scrim while taskbar is
                 stashed and bubbles / manage menu are open
             4 - check that three button nav works while
                 bubbles are expanded
Bug: 197139718
Change-Id: I94c4ecd07f81b2bad55c38525d60f493d3c1f9d8