Add MockSpellChecker and SpellCheckerTest.

This CL adds a minimal end-to-end test for Spell checker framework.
To this end, this CL introduces MockSpellChecker, which is inspired by
MockIme.

Bug: 166304720
Test: atest CtsInputMethodTestCases:SpellCheckerTest
Change-Id: I6d92ed0c7f7ca88011f6f5470798cfbc7ff3fa96
diff --git a/tests/inputmethod/Android.bp b/tests/inputmethod/Android.bp
index 6f07ac6..aa3be52 100644
--- a/tests/inputmethod/Android.bp
+++ b/tests/inputmethod/Android.bp
@@ -29,10 +29,13 @@
         "compatibility-device-util-axt",
         "ctstestrunner-axt",
         "CtsMockInputMethodLib",
+        "CtsMockSpellCheckerLib",
         "testng",
+        "kotlin-test",
     ],
     srcs: [
         "src/**/*.java",
+        "src/**/*.kt",
         "src/**/I*.aidl",
     ],
     aidl: {