Some code cleanup (4/n)

Now we can combine the 2 synchronization blocks inside
onAfterSessionRead().

Bug: 162386287
Test: atest StagedInstallTest
Change-Id: I018a36d7ef16aa7eb849b4f19f8fd339b5d9d635

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

@@ -1663,8 +1663,6 @@ public class PackageInstallerSession extends IPackageInstallerSession.Stub {
             if (!mShouldBeSealed || isStagedAndInTerminalState()) {
                 return;
             }
-        }
-        synchronized (mLock) {
             try {
                 sealLocked();