Winscope: Use a unique stableId for all layers and wm entries

Top level of the hierarchy contains the entry with a timestamp. When
navigating the timeline with arrow keys, entry loses focus since
the timestamp changes.

See: https://android-window-manager.teams.x20web.corp.google.com/winscope/winscope-staging-vishnun.html

Test: Open trace in winscope, click on entry & navigate timeline with arrow keys
Change-Id: Ifbd88da717354c66d96a05d25a9d619fa5209e26