updater_sample: fix payload spec tests

Test: junit4
Change-Id: Ia2f7475cfba01a65486bb0e5d0f3976304ca0969
Signed-off-by: Zhomart Mukhamejanov <zhomart@google.com>

updater_sample/tests/src/com/example/android/systemupdatersample/util/PayloadSpecsTest.java

@@ -65,7 +65,7 @@ public class PayloadSpecsTest {
         mTargetContext = InstrumentationRegistry.getTargetContext();
         mTestContext = InstrumentationRegistry.getContext();
 
-        mTestDir = mTargetContext.getFilesDir();
+        mTestDir = mTargetContext.getCacheDir();
         mPayloadSpecs = new PayloadSpecs();
     }