Use -target 8 for android_uiautomator

android_uiautomator is distributed as part of the SDK and should use
-target 8 for compatibility with old JDKs.

Bug: 150153790
Bug: 162522383
Test: javap -v -cp out/target/common/obj/JAVA_LIBRARIES/android_uiautomator_intermediates/classes.jar com.android.uiautomator.core.Configurator | grep "major version"
Change-Id: I6871ab5436232ad3bffff6bd26adf2de6b3c06ff

cmds/uiautomator/library/Android.bp

@@ -50,6 +50,7 @@ java_library_static {
         "android.test.runner",
         "junit",
     ],
+    java_version: "1.8",
 }
 
 java_library_static {