libsync: vendor_available

By setting vendor_available, the following may become true:

* a prebuilt library from this release may be used at runtime by
  in a later releasse (by vendor code compiled against this release).
  so this library shouldn't depend on runtime state that may change
  in the future.
* this library may be loaded twice into a single process (potentially
  an old version and a newer version). The symbols will be isolated
  using linker namespaces, but this may break assumptions about 1
  library in 1 process (your singletons will run twice).

Background:

This means that these modules may be built and installed twice --
once for the system partition and once for the vendor partition. The
system version will build just like today, and will be used by the
framework components on /system. The vendor version will build
against a reduced set of exports and libraries -- similar to, but
separate from, the NDK. This means that all your dependencies must
also mark vendor_available.

At runtime, /system binaries will load libraries from /system/lib*,
while /vendor binaries will load libraries from /vendor/lib*. There
are some exceptions in both directions -- bionic(libc,etc) and liblog
are always loaded from /system. And SP-HALs (OpenGL, etc) may load
/vendor code into /system processes, but the dependencies of those
libraries will load from /vendor until it reaches a library that's
always on /system. In the SP-HAL case, if both framework and vendor
libraries depend on a library of the same name, both versions will be
loaded, but they will be isolated from each other.

It's possible to compile differently -- reducing your source files,
exporting different include directories, etc. For details see:

https://android-review.googlesource.com/368372

None of this is enabled unless the device opts into the system/vendor
split with BOARD_VNDK_VERSION := current.

Bug: 33241851
Test: build and flash internal marlin
Test: m -j libsync
Test: build with BOARD_VNDK_VERSION := current
Change-Id: I5b23d2c1f41b842e5a9b7ea257921133b80c3f98