Add dummy string to Launcher3Tests resources to fix build error.

TL;DR;; since ag/656186 which moved the all-makefiles-under call to the bottom
of the launcher's Android.mk file, resources for test package
will not build without a dummy file. This change is necessary for the
test team to continuously run the android tests.

b/19566571

Change-Id: I4ff422ac019df4447348d953c5dd606d2a0dcc58