TradeFed: Add "test_config_template" flag in Android.bp

* Allow module owner to specify a test_config_template in Android.bp
* The rule goes:
  1. When "test_config" is set, Soong uses specified test config
  2. If 1 is not true, check if "AndroidTest.xml" exist in the
     directory, if so, use "AndroidTest.xml
  3. If 1 and 2 are not true, check if "test_config_template" is set.
     If so, use module specific template to generate test config
  4. Otherwise, use Soong default template for test config for autogen

Bug: 113359343
Test: make
Change-Id: I9fb4b2b266be9e0c7cf23da4a51e1c8ae67cd857