AAPT2: Process <java-symbols> and private symbol package

Need to introduce the idea of multiple levels of visibility to support <java-symbol>.

Public, Private, Undefined.

Public means it is accessible from outside and requires an ID assigned.
Private means that we explicitly want this to be a symbol (show up in R.java), but not visible
to other packages. No ID required.

Undefined is any normal resource. When --private-symbols is specified in the link phase,
these resources will not show up in R.java.

Change-Id: Icba89221e08e685dee7683786aa7112baf28c856