[Android] Fix that make_apk_test fails when out/ has both ARM and x86 bu...

...ilds

Both testCustomizeFile and testEmptyMode fail when out happens to have
both an ARM and an x86 build: in this case, the ARM native libraries will be around
and make_apk.py will generate both ARM and x86 APKs,
making the self.assertFalse(os.path.isfile('Example_1.0.0_arm.apk')) call fail.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1466