Make IMMS#shouldPreventImeStartupLocked() more readable

This is a mechanical refactoring with no behavior change for

  InputMethodManagerService#shouldPreventImeStartupLocked(),

which was added recently [1].

Most likely return-early pattern would be more readable and
maintainable for this kind of method.

There must be no observable behavior change in this CL.

 [1]: Id0aaa496ee46532d0e97c236df7e073947ababcd
      76a3c8c9

Bug: 234882948
Test: presubmit
Change-Id: Ic2d5164697523ec410b635b451fff1e4feb40830