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/Flags.java.template

@@ -6,10 +6,28 @@ import android.compat.annotation.UnsupportedAppUsage;
 /** @hide */
 public final class Flags \{
 {{- for item in flag_elements}}
+    {{ if library_exported }}
+    {{ if item.exported }}
     /** @hide */
     public static final String FLAG_{item.flag_name_constant_suffix} = "{item.device_config_flag}";
+    {{ endif }}
+    {{ else }}
+    /** @hide */
+    public static final String FLAG_{item.flag_name_constant_suffix} = "{item.device_config_flag}";
+    {{ endif }}
 {{- endfor }}
 {{ for item in flag_elements}}
+{{ if library_exported }}
+
+{{ if item.exported }}
+    @UnsupportedAppUsage
+    public static boolean {item.method_name}() \{
+        return FEATURE_FLAGS.{item.method_name}();
+    }
+{{ endif }}
+
+{{ else }}
+
 {{ -if not item.is_read_write }}
 {{ -if item.default_value }}
     @com.android.aconfig.annotations.AssumeTrueForR8
@@ -21,6 +39,7 @@ public final class Flags \{
     public static boolean {item.method_name}() \{
         return FEATURE_FLAGS.{item.method_name}();
     }
+{{ endif }}
 {{ endfor }}
 {{ -if is_test_mode }}
     public static void setFeatureFlags(FeatureFlags featureFlags) \{