Fix #removeChildSessionId (3/n)

Can't remove child from a sealed session.

Bug: 162286562
Test: Will be added in next CLs
Change-Id: I5fe20ede028f0e00e739316dab5c6a24b1546718

services/core/java/com/android/server/pm/PackageInstallerSession.java

@@ -3141,6 +3141,8 @@ public class PackageInstallerSession extends IPackageInstallerSession.Stub {
             }
 
             synchronized (mLock) {
+                assertCallerIsOwnerOrRootLocked();
+                assertPreparedAndNotSealedLocked("removeChildSessionId");
                 final int indexOfSession = mChildSessionIds.indexOfKey(sessionId);
                 if (session != null) {
                     session.setParentSessionId(SessionInfo.INVALID_ID);