DO NOT MERGE - Back-ported MockIme into CtsAutoFillServiceTestCases

Often CtsAutoFillServiceTestCases tests fail on new devices because the keyboard gets in the way.

This issue has been initially solved on Q by using a MockIme keyboard, which relies in a library
that was introduced on P. We already cherry-picked it to P, but for O we're copying the class
directly into Autofill's.

The code was copied-and-paste almost literally, except by the following changes:

- Used legacy annotations (instead of androidx)
- Changed copyright date to 2019
- Changed package name
- Removed requestShowSelf() and 'ime reset' calls
  (they're only available on P and not needed for our purposes)

Bug: 80079849
Bug: 129030690
Bug: 70969410
Bug: 126515980

Test: cts-tradefed run commandAndExit cts-dev -m CtsAutoFillServiceTestCases -t android.autofillservice.cts.LoginActivityTest#filterText_usingKeyboard
Test: cts-tradefed run commandAndExit cts-dev -m CtsAutoFillServiceTestCases  # sanity check

Change-Id: I55ff8913a6fb7ec2030d7f09a7bffd9cf889ca1c