aflags: Ensure <partition>/aconfig_flags.pb exists before reading

In some build targets/combinations aconfig_flags.pb
may not be generated.

For example in GSI only system/aconfig_flags.pb exists
and system-only updates won't have vendor/aconfig_flags.pb.

Test: m, aflags
Change-Id: I136345a8748bc6d80bd98e673f90e0a2ab380ab1

tools/aconfig/aconfig_device_paths/src/lib.rs

@@ -30,9 +30,11 @@ fn read_partition_paths() -> Vec<PathBuf> {
         .collect()
 }
 
-/// Determine all paths that contain an aconfig protobuf file.
+/// Determines all paths that contain an aconfig protobuf file,
+/// filtering out nonexistent partition protobuf files.
 pub fn parsed_flags_proto_paths() -> Result<Vec<PathBuf>> {
-    let mut result: Vec<PathBuf> = read_partition_paths();
+    let mut result: Vec<PathBuf> =
+        read_partition_paths().into_iter().filter(|s| s.exists()).collect();
 
     for dir in fs::read_dir("/apex")? {
         let dir = dir?;