aconfig: Add exported mode to aconfig Java library generation.

This commit adds a third codegen mode, _exported_, in addition to
the existing modes, production and test.

When codegen mode is _exported_, getters are generated _only_ for
flags marked as exported as well. Also the getters always look
up DeviceConfig values at runtime, and have a default value of
false.

This only implements exported mode for Java codegen, follow-up CLs
will support Rust and C++.

Test: atest aconfig.test
Bug: 311152507
Change-Id: Ie39379b40de072180e05d84c76361b24cc0e0d83

tools/aconfig/templates/FakeFeatureFlagsImpl.java.template

@@ -12,11 +12,23 @@ public class FakeFeatureFlagsImpl implements FeatureFlags \{
     }
 
 {{ for item in flag_elements}}
+{{ if library_exported }}
+
+{{ if item.exported }}
     @Override
     @UnsupportedAppUsage
     public boolean {item.method_name}() \{
         return getValue(Flags.FLAG_{item.flag_name_constant_suffix});
     }
+{{ endif }}
+
+{{ else }}
+    @Override
+    @UnsupportedAppUsage
+    public boolean {item.method_name}() \{
+        return getValue(Flags.FLAG_{item.flag_name_constant_suffix});
+    }
+{{ endif }}
 {{ endfor}}
     public void setFlag(String flagName, boolean value) \{
         if (!this.mFlagMap.containsKey(flagName)) \{