aconfig: Add exported mode to aconfig Java library generation.

This commit adds a third codegen mode, _exported_, in addition to
the existing modes, production and test.

When codegen mode is _exported_, getters are generated _only_ for
flags marked as exported as well. Also the getters always look
up DeviceConfig values at runtime, and have a default value of
false.

This only implements exported mode for Java codegen, follow-up CLs
will support Rust and C++.

Test: atest aconfig.test
Bug: 311152507
Change-Id: Ie39379b40de072180e05d84c76361b24cc0e0d83