ATest: GTest auto gen config support run-test-as.

Feature request from developer, support setting uid in makefile.

Bug: 113359343
Test: apply aosp/757329
      make -j hello_world_test_mk (w/ UID setting in config)
      make -j hello_world_test_mk2 (w/o UID setting in config)
      check test config of host's hello_world_test_mk (w/o UID setting in config)

Change-Id: I167c737098fa906e401769c6a8838f46b03bd33c
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 07e34e1..ddb217b 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -244,6 +244,7 @@
 LOCAL_RMTYPEDEFS:=
 LOCAL_RRO_THEME:=
 LOCAL_RTTI_FLAG:=
+LOCAL_RUN_TEST_AS :=
 LOCAL_SANITIZE:=
 LOCAL_SANITIZE_DIAG:=
 LOCAL_SANITIZE_RECOVER:=