IME: Fix InputMethodSystemServerTests for defer_show_soft_input_until_session_creation

With defer_show_soft_input_until_session_creation enabled, IMMS won't
show the IME unitl a session has been created.

Updates the tests to maintain a created session.

Bug: 339239657
Bug: 337766845
Test: atest FrameworksInputMethodSystemServerTests
Flag: TEST_ONLY
Change-Id: I6bd3da92cf953780dd62b1b29fd07c11bf511832