Make startTransition one-way

If shell is starting an existing transition, it doesn't need the
returned transition token so it can be an async call then it won't
block shell's thread to execute other operations.

If shell is starting a new transition, then use the new added 2-way
startNewTransition which is the same as the original path.

Bug: 248550757
Test: atest ShellTransitionTests
Test: CtsWindowManagerDeviceTestCases with shell transition
Change-Id: I5f64d19475d5b857a461775dd6f3002567e93ad8