Add a unit test for EditorInfo#targetInputMethodUser

This it a follow up CL to my previous CL [1], which introduced
EditorInfo#targetInputMethodUser.

Since EditorInfo#targetInputMethodUser is an @hide API, we cannot
directly its behavior in CTS.  To make sure that it can be transferred
via IPCs , this CL introduces a simple unit test for it in
FrameworksCoreTests.

 [1]: Ia7ea944438d69669ccdf9111b34ba400e786a602
      0f5eade4

Bug: 120744418
Test: atest FrameworksCoreTests:EditorInfoTest
Change-Id: I3e722b3ccbd2ff33c264d4d41aa63e704970e78d