Install VNDK snapshot libraries for system build

When BOARD_VNDK_VERSION := <VNDK version>, or
PRODUCT_EXTRA_VNDK_VERSIONS includes the needed <VNDK version> list,
the prebuilt VNDK libs in prebuilts/vndk/ directory will be
installed.

Each prebuilt VNDK module uses "vndk_prebuilt_shared" for shared
VNDK/VNDK-SP libs.

Following is the sample configuration of a vndk snapshot module:
vndk_prebuilt_shared {
    name: "libfoo",
    version: "27",
    vendor_available: true,
    vndk: {
        enabled: true,
    },
    arch: {
        arm64: {
            srcs: ["arm/lib64/libfoo.so"],
        },
        arm: {
            srcs: ["arm/lib/libfoo.so"],
        },
    },
}

The Android.bp for the snapshot modules will be auto-generated by a
script.

Bug: 38304393
Bug: 65377115
Bug: 68123344
Test: set BOARD_VNDK_VERSION := 27
      copy a snapshot for v27
      build with make command

Change-Id: Ib93107530dbabb4a24583f4d6e4f0c513c9adfec
diff --git a/android/config.go b/android/config.go
index a4624c7..0eebb5f 100644
--- a/android/config.go
+++ b/android/config.go
@@ -632,11 +632,12 @@
 	return "vendor"
 }
 
-func (c *deviceConfig) CompileVndk() bool {
-	if c.config.ProductVariables.DeviceVndkVersion == nil {
-		return false
-	}
-	return *c.config.ProductVariables.DeviceVndkVersion == "current"
+func (c *deviceConfig) VndkVersion() string {
+	return String(c.config.ProductVariables.DeviceVndkVersion)
+}
+
+func (c *deviceConfig) ExtraVndkVersions() []string {
+	return c.config.ProductVariables.ExtraVndkVersions
 }
 
 func (c *deviceConfig) BtConfigIncludeDir() string {