Add LOCAL_NDK_VERSION.

Used to select between releases of the NDK (r10, r11, etc).

Some of the libraries we use in Android come as prebuilts from
google3, which are most likely built against an older NDK than what
we have in the platform. Since things may not be ABI compatible
release to release, we need to keep legacy versions accessible for
those to keep building.

Change-Id: Ia3df42fa6f3a7bd5108ff6bbb0f2ca06239c3be9

core/binary.mk

@@ -66,14 +66,11 @@ my_ndk_sysroot :=
 my_ndk_sysroot_include :=
 my_ndk_sysroot_lib :=
 ifdef LOCAL_SDK_VERSION
-  ifdef LOCAL_NDK_VERSION
-    $(error $(LOCAL_PATH): LOCAL_NDK_VERSION is now retired.)
-  endif
   ifdef LOCAL_IS_HOST_MODULE
     $(error $(LOCAL_PATH): LOCAL_SDK_VERSION cannot be used in host module)
   endif
-  my_ndk_source_root := $(HISTORICAL_NDK_VERSIONS_ROOT)/current/sources
-  my_ndk_sysroot := $(HISTORICAL_NDK_VERSIONS_ROOT)/current/platforms/android-$(LOCAL_SDK_VERSION)/arch-$(TARGET_$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)
+  my_ndk_source_root := $(HISTORICAL_NDK_VERSIONS_ROOT)/$(LOCAL_NDK_VERSION)/sources
+  my_ndk_sysroot := $(HISTORICAL_NDK_VERSIONS_ROOT)/$(LOCAL_NDK_VERSION)/platforms/android-$(LOCAL_SDK_VERSION)/arch-$(TARGET_$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)
   my_ndk_sysroot_include := $(my_ndk_sysroot)/usr/include
 
   # x86_64 and and mips64 are both multilib toolchains, so their libraries are

core/clear_vars.mk

@@ -190,6 +190,7 @@ LOCAL_NOTICE_FILE:=
 # Used to replace the installed file of a presigned prebuilt apk in PDK fusion build,
 # to avoid installing the presigned apks with classes.dex unstripped.
 LOCAL_REPLACE_PREBUILT_APK_INSTALLED:=
+LOCAL_NDK_VERSION:=current
 
 # arch specific variables
 LOCAL_SRC_FILES_$(TARGET_ARCH):=