Add screenshot support in winscope.

Can upload .png files to be displayed in SR viewer - only if SR trace not also uploaded.
Can take a screenshot when taking a WM/SF dump in via winscope collection UI.
Cannot upload a .png file or take just a screenshot without also uploading a winscope trace.

Fixes: 274910593
Test: npm run test:unit:ci
Change-Id: I9e1712cd2acb354c0a6e030306c3c35c4efcd1dd