inputflinger_tests: Put `FakeInputReaderPolicy` 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: Ibbe27ced5adec2186decd8fb1c359b8ed4cbf870
diff --git a/services/inputflinger/tests/Android.bp b/services/inputflinger/tests/Android.bp
index e0466d5..611fe3d 100644
--- a/services/inputflinger/tests/Android.bp
+++ b/services/inputflinger/tests/Android.bp
@@ -41,6 +41,7 @@
         "BlockingQueue_test.cpp",
         "EventHub_test.cpp",
         "FakeEventHub.cpp",
+        "FakeInputReaderPolicy.cpp",
         "FakePointerController.cpp",
         "FocusResolver_test.cpp",
         "InputProcessor_test.cpp",