Add IVold::destroyDsuMetadataKey()

destroyDsuMetadataKey() / destroy_dsu_metadata_key() calls
android::gsi::GetDsuMetadataKeyDir() to query the DSU metadata
encryption key dir and destroy the key.
This releases the resource and allows consecutive DSU installations to
use the same key *directory*, but not the same key *blob*.

Bug: 168571434
Test: 1. Install a DSU system.
  2. Boot the DSU system and reboot back to the host system.
  3. Wipe the DSU installation.
  4. DSU metadata key dir /metadata/vold/metadata_encryption/dsu/dsu is
     destroyed.
Change-Id: Ib851177315a5a266807f46ccfd446de1848232cf
diff --git a/VoldNativeService.h b/VoldNativeService.h
index f10bf5f..9914879 100644
--- a/VoldNativeService.h
+++ b/VoldNativeService.h
@@ -159,6 +159,8 @@
             const ::android::os::incremental::IncrementalFileSystemControlParcel& control,
             bool enableReadLogs) override;
     binder::Status bindMount(const std::string& sourceDir, const std::string& targetDir) override;
+
+    binder::Status destroyDsuMetadataKey(const std::string& dsuSlot) override;
 };
 
 }  // namespace vold