feat(keystore): mirror lifecycle via maintenance

Hook the keystore2 daemon's android.security.maintenance binder (hosted
by the same process, reached by the already-injected native hook) so
synthetic key state follows real key-lifecycle events:

- clearNamespace(APP) purges synthetic keys for the uid and their grants.
- deleteAllKeys() clears all synthetic keys and grants.
- migrateKeyNamespace() re-keys the synthetic entry, preserving material,
  chain, and grants.

Pure side-effect hook: every handled transaction mutates only our own
synthetic state, then returns ContinueAndSkipPost so the real keystore2
still performs the real operation. Unhandled codes pass through, so real
key lifecycle is never disturbed. Pre-empts delete-then-read and
clearNamespace coherence probes (Phase 9 Change 4).

Adds a minimal IKeystoreMaintenance compile stub for the descriptor;
transaction codes resolve reflectively on-device.

Refs Phase 9 .omc/plans/tee-fingerprint-phase-9-grant-plane-coherence.md

stub/src/main/java/android/security/maintenance/IKeystoreMaintenance.java

@@ -0,0 +1,22 @@
+package android.security.maintenance;
+
+import android.os.IBinder;
+
+/**
+ * Compile-time stub for the hidden keystore2 maintenance binder
+ * ({@code android.security.maintenance.IKeystoreMaintenance}).
+ *
+ * <p>This module is a {@code compileOnly} dependency, so the real framework class
+ * (which carries the actual {@code TRANSACTION_*} codes) is loaded at runtime. We
+ * only need the {@link #DESCRIPTOR} token to parse the transaction parcel and the
+ * inner {@code Stub} class so {@code getTransactCode} can reflect the real codes.
+ */
+public interface IKeystoreMaintenance {
+    String DESCRIPTOR = "android.security.maintenance.IKeystoreMaintenance";
+
+    class Stub {
+        public static IKeystoreMaintenance asInterface(IBinder b) {
+            throw new UnsupportedOperationException("STUB!");
+        }
+    }
+}