soong_zip builds a list of files to zip early and then starts
zipping them all. If a directory being zipped is concurrently
modified, a file that existed when soong_zip started may not
still exist. Add a flag that continues when an expected file
does not exist. Print a warning, since this should be rare
in normal usages but is a sign of a problem if it happens
regularly.
Test: zip_test.go
Test: m checkbuild
Test: m platform
Change-Id: I78426fe66fded8528ddd436c0f71a7442183cfeb
4be8f9e2a3