Introduce a .winscope extension for all winscope files

This extension will be used to better integrate winscope traces and ABT,
allowing trace files to be opened directly from ABT or buganizer

Bug: 192663722
Bug: 162813517
Test: (for winscope) build winscope, record traces, download traces, check if traces have the .winscope extension. (for flicker) run atest Flickertests. (for abt) on new extension, check if .winscope files show as "open in winscope" on buganizer

Change-Id: I969a42e272fec2f6e6e27173e9475e6142faefa6