Use FileDescriptorPtr to implement async reads in verify stage

During FileSystemVerify stage, update_engine needs to read from source
or target partition to verify hashes && write verity. Previously we use
brillow's file stream to implement async reads. WIth Virtual AB
Compression, reading from target partition must go through libsnapshot's
interface(FileDescriptorPtr). So we replace brillo::FileStream with
FileDescriptorPtr for ease of integrating with VABC.

Test: serve an OTA update, verify: slot switch resume, regular resume

Change-Id: Id8531757468f60e3e21667b7761b83f7c2af2dbf
diff --git a/payload_consumer/file_descriptor.h b/payload_consumer/file_descriptor.h
index fb07ff0..faebcc1 100644
--- a/payload_consumer/file_descriptor.h
+++ b/payload_consumer/file_descriptor.h
@@ -111,6 +111,7 @@
 class EintrSafeFileDescriptor : public FileDescriptor {
  public:
   EintrSafeFileDescriptor() : fd_(-1) {}
+  ~EintrSafeFileDescriptor();
 
   // Interface methods.
   bool Open(const char* path, int flags, mode_t mode) override;