Rename RWC#getLaunchRootTask

...in order to stop the "launch-root-task" term being widely used.

The "launch root task" can now just refer to the root task that
is explicitly specified externally, either from the
ActivityOption#setLaunchRootTask that sets from caller app
or the TDA#mLaunchRootTasks that sets from WM Shell.

No logical changes in this CL.

Bug: 184806710
Test: wm presubmit
Change-Id: I6e31ffd5285234f59d8b0bf3e7e2e5de09a8ecbc