build: use class path bases, not strings

scripts/slave/recipe_modules/emulator/config.py

 from recipe_engine.config import config_item_context, ConfigGroup
 from recipe_engine.config import Single, Static
 from recipe_engine.config_types import Path
 
-def BaseConfig(**_kwargs):
+def BaseConfig(CHECKOUT_PATH, **_kwargs):
   return ConfigGroup(
-    install_emulator_deps_path = Static(Path('[CHECKOUT]', 'build', 'android',
-                                             'install_emulator_deps.py')),
-    avd_script_path = Static(Path('[CHECKOUT]', 'build', 'android', 'avd.py'))
+    install_emulator_deps_path = Static(
+        CHECKOUT_PATH.join('build', 'android', 'install_emulator_deps.py')),
+    avd_script_path = Static(CHECKOUT_PATH.join('build', 'android', 'avd.py')),
   )
 
 config_ctx = config_item_context(BaseConfig)
 
 @config_ctx()
 def base_config(c):
   pass