Copy boot*img files to IMAGES/ in target_files.zip

The following files will be added to the soong-built zip file after this
CL
1. boot.img
2. init_boot.img
3. vendor_boot.img

Implementation details
- Refactor the IMAGES/ dir generation to `copyImagesToTargetZip`
- Use VisitDirectDepsProxyWithTag(filesystemDepTag) to visit filesystem
  deps (e.g. system) and bootimg deps.
- Set the Stem property in the autogenerated soong bootimg modules

Test: Built the zip file locally and verified that boot*img files are
copied (although they are not bit identical to the make equivalents)
Bug: 388635097

Change-Id: Ia7138761146e93a268eecb9dc9bf4d579445980b