Avoid NPE around mReusedTask of ActivityStarter

Symptom:
System crash by NPE is observed on ActivityStarter
during phone boot-up.

Detail and sample:
ActivityStarter has possibility to be called itself recursively
by calling TaskRecord$performClearTaskLocked().
Then class variable of mReusedTask is initialized to null
after the call, because the recursive call isn't guarded by
synchronized block.
Then NPE crash occurs on next statement.

Solutions:
Avoid to use the class variable on null possibility statement.

Bug: 32361138

Change-Id: Iaea3e066a6f7134fcae4338ff864bb236241194c

services/core/java/com/android/server/am/ActivityStarter.java

@@ -1540,9 +1540,9 @@ class ActivityStarter {
                 == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK)) {
             // The caller has requested to completely replace any existing task with its new
             // activity. Well that should not be too hard...
+            intentActivity.task.performClearTaskLocked();
+            intentActivity.task.setIntent(mStartActivity);
             mReuseTask = intentActivity.task;
-            mReuseTask.performClearTaskLocked();
-            mReuseTask.setIntent(mStartActivity);
             // When we clear the task - focus will be adjusted, which will bring another task
             // to top before we launch the activity we need. This will temporary swap their
             // mTaskToReturnTo values and we don't want to overwrite them accidentally.