Include headers and props to VNDK snapshot

For all vndk snapshot libraries, header files exported by the libraries
will be included to the snapshot. Android.bp will contain necessary
information to link against/install vndk snapshot libraires:
export_include_dirs, export_system_include_dirs, export_flags, and
relative_install_path.

Bug: 132818174
Test: 1) m nothing && mv out/soong/build.ninja /tmp && m nothing &&
         diff -u -u out/soong/build.ninja /tmp/build.ninja
Test: 2) VNDK_SNAPSHOT_BUILD_ARTIFACTS=true \
           development/vndk/snapshot/build.sh
Test: 3) development/vndk/snasphot/update.py
Test: 4) see contents of Android.bp and include directories
Change-Id: I791ab181545eb483242b04446afd40958bbb2b17

android/variable.go

@@ -261,7 +261,8 @@ type productVariables struct {
 
 	PgoAdditionalProfileDirs []string `json:",omitempty"`
 
-	VndkUseCoreVariant *bool `json:",omitempty"`
+	VndkUseCoreVariant         *bool `json:",omitempty"`
+	VndkSnapshotBuildArtifacts *bool `json:",omitempty"`
 
 	BoardVendorSepolicyDirs      []string `json:",omitempty"`
 	BoardOdmSepolicyDirs         []string `json:",omitempty"`