Improve long press menu readability

When media cannot be dismissed, hide the "Hide" button completely and make
the "Cancel" button solid. Also update the top text to be medium weight
so it is more readable on busy backgrounds.

Fixes: 231397561
Test: manual - video in bug
Test: atest com.android.systemui.media
Change-Id: I1f9341d84102aaf6634745c42cf23ce572136954