Differentiate between exported and internal sdk members

Internal sdk members are used by an sdk member but not exported by the
sdk. The exported sdk members are those listed explicitly in one of the
sdk member list properties, e.g. java_header_libs.

The prebuilts of an internal sdk member use a unique name so that they
do not clash with the source module. The use of the module internally
is an implementation detail that must not have any effect outside the
snapshot. Having the same name as the source module could cause it to
override the source module, hence why it needs a unique name.

Similarly, they are marked as private so as to prevent their accidental
use from outside the snapshot.

Bug: 142940300
Test: m nothing
Change-Id: Id5364b410be0592f65666afb3e40e9d3f020251c