Some cleanup (3/n)

Bug: 166694095
Test: atest StagedInstallTest
Change-Id: Ibbf7f77ca512498c7b4b3080a11174ac96bb932c
diff --git a/services/core/java/com/android/server/pm/PackageInstallerSession.java b/services/core/java/com/android/server/pm/PackageInstallerSession.java
index 41eb3bc..4fe72c2 100644
--- a/services/core/java/com/android/server/pm/PackageInstallerSession.java
+++ b/services/core/java/com/android/server/pm/PackageInstallerSession.java
@@ -575,8 +575,7 @@
         @Override
         public void installSession(IntentSender statusReceiver) {
             assertCallerIsOwnerOrRootOrSystemLocked();
-            Preconditions.checkArgument(!hasParentSessionId()); // Don't allow installing child
-                                                                // sessions
+            assertNotChildLocked("StagedSession#installSession");
             Preconditions.checkArgument(isCommitted() && isSessionReady());
 
             // Since staged sessions are installed during boot, the original reference to status
@@ -750,7 +749,6 @@
         public void verifySession() {
             assertCallerIsOwnerOrRootOrSystemLocked();
             Preconditions.checkArgument(isCommitted());
-            Preconditions.checkArgument(isStaged());
             Preconditions.checkArgument(!mSessionApplied && !mSessionFailed);
             verify();
         }