Use pre-computed top process for updating previous process

There was a computed field with the same meaning, so it is
unnecessary to traverse the entire hierarchy to find again.
The traversal method is also removed because no one uses it
anymore.

Moved updatePreviousProcess from RWC to ATMS because it
updates the fields of ATMS.

Bug: 163976519
Test: CtsWindowManagerDeviceTestCases
Change-Id: I9177cb34b0456e5c60542fd1bac40b884d8db3a7