aconfig: remove UnsupportedAppUsage from java exported mode

UnsupportedAppUsage was added to support cts test access issue. For new
exported mode, if the cts tests code with exported flag, there won't be access issue. So this annotation is not needed. Removing this annotation also removes the dependency on this annotation in google3 code.

Test: atest aconfig.test
Bug: 311152507
Bug: 303773055
Change-Id: I8c190e69cdb514af7992ccca7fea152c917047dc