Add setFlag and resetAll in FeatureFlags test mode

Add methods setFlag and resetAll in FeatureFlags in test mode. For the
injection usecase, user will use the interface FeatureFlags in the code
to control the flags.

Add tests for test mode.

Bug: 280833463
Test: Atest AconfigJavaHostTest --host
Change-Id: Ib59ba35a9011a6400af42fc9c283d37193577997

tools/aconfig/Android.bp

@@ -101,7 +101,7 @@ java_aconfig_library {
 android_test {
     name: "aconfig.test.java",
     srcs: [
-        "tests/**/*.java",
+        "tests/AconfigTest.java",
     ],
     manifest: "tests/AndroidManifest.xml",
     certificate: "platform",
@@ -113,6 +113,25 @@ android_test {
     test_suites: ["device-tests"],
 }
 
+java_aconfig_library {
+    name: "aconfig_host_test_java_library",
+    aconfig_declarations: "aconfig.test.flags",
+    host_supported: true,
+    test: true,
+}
+
+java_test_host {
+    name: "AconfigJavaHostTest",
+    srcs: [
+        "tests/AconfigHostTest.java",
+    ],
+    static_libs: [
+        "aconfig_host_test_java_library",
+        "junit",
+    ],
+    test_suites: ["general-tests"],
+}
+
 // integration tests: C++
 
 cc_aconfig_library {