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
Change-Id: Ie9526e3755a6e7d142b377d5dd85a1bc60ffabc5