Allow test to specify whether or not to auto-generate test config

Attribute `auto_gen_config` is added to test modules.
Test config will be generated if:
the attribute is not set and AndroidTest.xml doesn't exists
or
the attribute is set to true, whether or not AndroidTest.xml exists.

Test config will NOT be auto-generated if:
the attribute is not set and AndroidTest.xml exists
or
the attribute is set to false, whether or not AndroidTest.xml exists.

Bug: 141684102
Test: build test module with auto_gen_config set to true
Change-Id: I64fb003a83d8c32a967835e5f8d12fe4476043be