Add property Exportable to aconfig_declarations

Only when aconfig_declarations explicitly set this property true,
its flags will be repackaged.

If a java_aconfig_library has mode "exported", its corresponding
aconfig_declarations must have exportable prop set to true.

Test: Added unit tests and CI

Bug: 310504781

Ignore-AOSP-First: this change will be cherry picked to aosp
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e4527f3010b3fd6f33948a15a9b4fdc944da34f0)
Merged-In: Ie9526e3755a6e7d142b377d5dd85a1bc60ffabc5
Change-Id: Ie9526e3755a6e7d142b377d5dd85a1bc60ffabc5

aconfig/codegen/java_aconfig_library_test.go

@@ -176,6 +176,7 @@ func testCodegenMode(t *testing.T, bpMode string, ruleMode string) {
 				name: "my_aconfig_declarations",
 				package: "com.example.package",
 				srcs: ["foo.aconfig"],
+				exportable: true,
 			}
 
 			java_aconfig_library {