Add mount_point property to android_filesystem

The motivation is to support vendor images for microdroid, like the real
vendor images having "/vendor" as their mount point. This will help add
vendor_file_contexts easily.

Bug: 306313100
Test: manually build an image with file_contexts
Change-Id: I2e4bbf108eaa1da4f310ebb4099c4d1b42096436
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index 023c69a..f2efd46 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -104,6 +104,9 @@
 	// When set, passed to mkuserimg_mke2fs --mke2fs_uuid & --mke2fs_hash_seed.
 	// Otherwise, they'll be set as random which might cause indeterministic build output.
 	Uuid *string
+
+	// Mount point for this image. Default is "/"
+	Mount_point *string
 }
 
 // android_filesystem packages a set of modules and their transitive dependencies into a filesystem
@@ -332,7 +335,7 @@
 	}
 
 	addStr("fs_type", fsTypeStr(f.fsType(ctx)))
-	addStr("mount_point", "/")
+	addStr("mount_point", proptools.StringDefault(f.properties.Mount_point, "/"))
 	addStr("use_dynamic_partition_size", "true")
 	addPath("ext_mkuserimg", ctx.Config().HostToolPath(ctx, "mkuserimg_mke2fs"))
 	// b/177813163 deps of the host tools have to be added. Remove this.