Mark as 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: 36426473
Bug: 36079834
Test: Android-aosp_arm.mk is the same before/after
Test: build.ninja is the same before/after
Test: build-aosp_arm.ninja is the same before/after
Test: attempt to compile with BOARD_VNDK_VERSION := current
Change-Id: Ia4eb5378d941033b07673daf682e66051cd3c075

Android.bp

@@ -3,6 +3,7 @@
 cc_library_headers {
     name: "libhardware_headers",
     export_include_dirs: ["include"],
+    vendor_available: true,
 }
 
 cc_library_shared {
@@ -17,6 +18,7 @@ cc_library_shared {
     header_libs: ["libsystem_headers"],
     cflags: ["-DQEMU_HARDWARE"],
     export_include_dirs: ["include"],
+    vendor_available: true,
 }
 
 subdirs = [