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.cc b/payload_consumer/file_descriptor.cc
index 6101c68..5b27c38 100644
--- a/payload_consumer/file_descriptor.cc
+++ b/payload_consumer/file_descriptor.cc
@@ -29,6 +29,12 @@
 
 namespace chromeos_update_engine {
 
+EintrSafeFileDescriptor::~EintrSafeFileDescriptor() {
+  if (IsOpen()) {
+    Close();
+  }
+}
+
 bool EintrSafeFileDescriptor::Open(const char* path, int flags, mode_t mode) {
   CHECK_EQ(fd_, -1);
   return ((fd_ = HANDLE_EINTR(open(path, flags, mode))) >= 0);