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

android/variable.go

@@ -192,6 +192,8 @@ type productVariables struct {
 
 	DeviceKernelHeaders []string `json:",omitempty"`
 	DistDir             *string  `json:",omitempty"`
+
+	ExtraVndkVersions []string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {