Enable IME test: "show and hide IME layer"

This test was previously disabled because it was flaky. This CL re-enables the test.

 To check if the IME works correctly the flicker tests check:
 (1) IME layer is invisible in the beginning
 (2) IME layer is displayed
 (3) once shown, IME layer is visible in all following entries

Test: atest FlickerTests:OpenImeWindowTest
Change-Id: I89abb369904fc39733f4aef8038e2ad1be2607d9