Add install_symlink soong module type

This can be used to install symlinks to arbitrary locations/targets
on the device. Used to replace a make-built symlink.

Bug: 205632228
Test: built and ran the emulator observed the /system/bin/hwservicemanager symlink is still there
Change-Id: I6df922c8d919e6d56fa79702815a89c98f4d65ed

etc/Android.bp

@@ -14,10 +14,12 @@ bootstrap_go_package {
     srcs: [
         "prebuilt_etc.go",
         "snapshot_etc.go",
+        "install_symlink.go",
     ],
     testSrcs: [
         "prebuilt_etc_test.go",
         "snapshot_etc_test.go",
+        "install_symlink_test.go",
     ],
     pluginFor: ["soong_build"],
 }