Renamed some getXXXStack methods to getRootXXXTask (87/n)

Also stacks are tasks now so this is "techincally" correct.
Also, stacks are going to be removed soon, so this CL helps reduce
size of future CLs.

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ibc47b63f19fd154b9162264f0ad1c9259a3e5d56
diff --git a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index 9f4c581..31b7c68 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -4080,7 +4080,7 @@
         try {
             synchronized (mGlobalLock) {
                 final ActivityStack stack =
-                        mRootWindowContainer.getDefaultDisplay().getSplitScreenPrimaryStack();
+                        mRootWindowContainer.getDefaultDisplay().getRootSplitScreenPrimaryTask();
                 if (stack == null) {
                     Slog.w(TAG, "dismissSplitScreenMode: primary split-screen stack not found.");
                     return;
@@ -4121,7 +4121,7 @@
         try {
             synchronized (mGlobalLock) {
                 final ActivityStack stack =
-                        mRootWindowContainer.getDefaultDisplay().getPinnedStack();
+                        mRootWindowContainer.getDefaultDisplay().getRootPinnedTask();
                 if (stack == null) {
                     Slog.w(TAG, "dismissPip: pinned stack not found.");
                     return;