Move wm component initialization to shell main thread

- Previously, mWMComponent.init() would make a blocking call to WMShell
  to initialize on the shell main thread.  We're expanding that to
  include the construction of the shell objects as well to provide
  a little more consistency and predictability on the shell side
  (ie. fetching some statics may seem safe, but actually aren't).
  We're continuing to recommending that ctors don't include any
  initialization logic though

Bug: 181979527
Test: Verify with trace that shell init happens on the
      shell main thread
Test: atest SystemUITests
Test: atest WMShellUnitTests

Change-Id: I60559a47e00e91d34be7d4e94f4f3e041fdc590d
Signed-off-by: Winson Chung <winsonc@google.com>