Add upload-only mode and manual build-started-time flag.

This involved refactoring much of the main method in soong_ui/main.

Test: b build libcore:all
Test: use the build-time started flag and verify via printf that
it's the same across upload.go and metrics.SetBuildDateTimestamp()

Change-Id: Id7fe256337e8ee6c40542eba662c0eadb38e9674