Test cursor after launching another activity

There is a regression started from CL[1] since checking
editor logic isn't accurate enough and fixed by CLs[2,3].

To prevent such regression in the future, add a test to
verify cursor position after launching another activity.

[1]: I45a9814d812ad906f417c24200fd4219959e2423
[2]: I1ef3d341af9d473d94d52fd1890deafbae2bc9e1
[3]: I022ad658844142ff4a4cf3b91953013f2bfbb58a

Bug: 159238365
Test: verify this test fails in issue ROM
Test: atest InputMethodServiceTest
Change-Id: I8ecd9db589bad8b5b003391f60e5b5fa857545c0