Rename time to event in Soong metrics system.

Several structs and files are named time which records the total
execution of a traceable event. Soon, resource information of an
executed process is collected during a build event so a more generic
name is applied.

Bug: b/169453825
Test: go test and m nothing. Checked the soong_metrics protobuf.
Change-Id: I65a782603530d51018d6cc1192c18cfeef7566a0