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
diff --git a/fsgen/boot_imgs.go b/fsgen/boot_imgs.go
index 914d35b..889a4c2 100644
--- a/fsgen/boot_imgs.go
+++ b/fsgen/boot_imgs.go
@@ -80,6 +80,7 @@
 			Security_patch:     securityPatch,
 			Dtb_prebuilt:       dtbPrebuilt,
 			Cmdline:            cmdline,
+			Stem:               proptools.StringPtr("boot.img"),
 		},
 		&struct {
 			Name *string
@@ -133,6 +134,7 @@
 			Dtb_prebuilt:       dtbPrebuilt,
 			Cmdline:            cmdline,
 			Bootconfig:         vendorBootConfigImg,
+			Stem:               proptools.StringPtr("vendor_boot.img"),
 		},
 		&struct {
 			Name *string
@@ -180,6 +182,7 @@
 			Avb_private_key:    avbInfo.avbkeyFilegroup,
 			Avb_rollback_index: avbInfo.avbRollbackIndex,
 			Avb_algorithm:      avbInfo.avbAlgorithm,
+			Stem:               proptools.StringPtr("init_boot.img"),
 		},
 		&struct {
 			Name *string