Introduce make var ANDROID_PUBLIC_EXPORTABLE_STUBS

ANDROID_PUBLIC_STUBS currently has multiple usages in the build, in
multiple .mk files.

Instead of modifying the current functionality of ANDROID_PUBLIC_STUBS
by replacing its value from "android_stubs_current" to
"android_stubs_current_exportable", this change introduces
ANDROID_PUBLIC_EXPORTABLE_STUBS, which evaluates to
"android_stubs_current_exportable" and used only when generating the
`full_target` in `development/build/Android.mk`.

Test: patch in git_main, lunch aosp_arm-ap31-eng && m sdk dist && inspect android.jar outputs
Bug: 323261972
Change-Id: Ic2b03a5a4afdefb459d89f8104916446599eaf31