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
Merged-In: I6871ab5436232ad3bffff6bd26adf2de6b3c06ff
(cherry picked from commit 51387a02)