inputflinger_tests: Put `InputMapperTest` in its own file

I would like to be able to put automated tests for the new
`TouchpadEventMapper` in their own file, rather than
InputReader_tests.cpp. To do this I'll need some of the test utilities
in their own files, too.

Bug: 251196347
Test: atest inputflinger_tests
Change-Id: Ie524adb3431e4a0c59efe75be82abddacd676c21
diff --git a/services/inputflinger/tests/Android.bp b/services/inputflinger/tests/Android.bp
index 2a8b359..53d821f 100644
--- a/services/inputflinger/tests/Android.bp
+++ b/services/inputflinger/tests/Android.bp
@@ -44,6 +44,7 @@
         "FakeInputReaderPolicy.cpp",
         "FakePointerController.cpp",
         "FocusResolver_test.cpp",
+        "InputMapperTest.cpp",
         "InputProcessor_test.cpp",
         "InputProcessorConverter_test.cpp",
         "InputDispatcher_test.cpp",