aconfig: flag java lib enable optimization

This change adds annotation in the generated code. If the flag is read
only, annotations @com.android.aconfig.annotations.AssumeTrueForR8 or
@com.android.aconfig.annotations.AssumeFalseForR8 will be added in to
the corresponding methods in the interface FeatureFlags, and class
Flags.

Test: atest aconfig aconfig.test.java
Bug: 295328116
Change-Id: Ic1b62a86f0945a5e72ae2c4987cce77fe3f172e9

tools/aconfig/templates/Flags.java.template

@@ -5,6 +5,13 @@ public final class Flags \{
     public static final String FLAG_{item.flag_name_constant_suffix} = "{item.device_config_flag}";
 {{- endfor }}
 {{ for item in class_elements}}
+{{ -if not item.is_read_write }}
+{{ -if item.default_value }}
+    @com.android.aconfig.annotations.AssumeTrueForR8
+{{ -else }}
+    @com.android.aconfig.annotations.AssumeFalseForR8
+{{ -endif- }}
+{{ endif }}
     public static boolean {item.method_name}() \{
         return FEATURE_FLAGS.{item.method_name}();
     }
@@ -17,7 +24,7 @@ public final class Flags \{
     public static void unsetFeatureFlags() \{
         Flags.FEATURE_FLAGS = null;
     }
-{{ endif}}
+{{ endif }}
     private static FeatureFlags FEATURE_FLAGS{{ -if not is_test_mode }} = new FeatureFlagsImpl(){{ -endif- }};
 
 }