Optimize memory used for creating saved recording notification

Updated the SavedRecording object to save the icon for notification
thumbnail, instead of bitmap, as bitmap is later converted to Icon.
Updated the size of the thumbnail to match the size required for the
notification, to avoid multiple resizings.

memory profile -
before http://screen/mDN2iC5SsFDYdi5 29MB
after http://screen/AxhE9HoAWqwGyqG 10MB

Test: manually tested
Fixes: http://b/278223891
Flag: None
Change-Id: I4f65a8ed542819d741968ed0e44d430307369fd0