Build System: Add variable for copying test config

If LOCAL_TEST_CONFIG is set, it will be the default test config
and ignore the local AndroidTest.xml.

BUG: 110982517
Test: vi art/tools/ahat/Android.mk
      set LOCAL_TEST_CONFIG := ahat_test.xml
      add ahat_test.xml in local
      m -j ahat;
      Then, out/host/linux-x86/testcases/ahat/ahat.config
      and ahat_test.xml should be the same.

Change-Id: I2f4ef735fca7d523bb637a18075512bdebf9826f