Add avb info of bootimages in misc_info.txt

Make built $ANDROID_PRODUCT_OUT/misc_info.txt contains the avb
information of the installed boot images. This info overlaps with the
contents of the buildPropFile of the `bootImg` modules, but
- Only some of the entries are written to misc_info.txt
- misc_info.txt contains the "partition qualified" key. e.g.
  avb_init_boot_algorithm and not avb_algorithm

This CL adds a secondary `propFileForMiscInfo` to the build actions
of bootimg. This file will be propagated to android_device via a
provider and cat'd to the misc_info.txt created by android_device

(There are still a lot of diffs between Make and Soong misc_info.txt)

Bug: 398036609
Test: Built Soong's misc_info.txt
Change-Id: I0c49ebaf7a2bac934dca05bcdab65b8521b891ed
diff --git a/fsgen/boot_imgs.go b/fsgen/boot_imgs.go
index 58ebcc4..0ba0a90 100644
--- a/fsgen/boot_imgs.go
+++ b/fsgen/boot_imgs.go
@@ -69,6 +69,7 @@
 	ctx.CreateModule(
 		filesystem.BootimgFactory,
 		&filesystem.BootimgProperties{
+			Boot_image_type:             proptools.StringPtr("boot"),
 			Kernel_prebuilt:             proptools.StringPtr(":" + kernelFilegroupName),
 			Header_version:              proptools.StringPtr(partitionVariables.BoardBootHeaderVersion),
 			Partition_size:              partitionSize,