Enable/disable current targets via soong var instead of inline.

Add a soong variable to enable/disable all targets that build
against the current SDK. This makes it easy to enable or disable
those targets all together. It also makes it much easier to
automerge Android.bp changes from AOSP to mainline-prod without
merge conflict, because the build target definitions are always
the same, only the variable at the top of the file changes.

Bug: 167645754
Test: m NetworkStackApiStableShims NetworkStackApiCurrentShims succeeds
Change-Id: Ie0e7d4f73efcf1942aadf8b9d67e4de914307c09
Merged-In: Ie0e7d4f73efcf1942aadf8b9d67e4de914307c09