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/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index 61e1e54..1d6225f 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -139,6 +139,8 @@
     void setIncFsMountOptions(in IncrementalFileSystemControlParcel control, boolean enableReadLogs);
     void bindMount(@utf8InCpp String sourceDir, @utf8InCpp String targetDir);
 
+    void destroyDsuMetadataKey(@utf8InCpp String dsuSlot);
+
     const int ENCRYPTION_FLAG_NO_UI = 4;
 
     const int ENCRYPTION_STATE_NONE = 1;