android-tools: Avoid installing avbtool twice and update the description

The last release (version 33.0.3p1 [0], added in 28609af5c7) started to
include avbtool and mkdtboimg [1].
This is just a minor fixup to update the package description and stop
installing avbtool twice:
$ sha256sum /nix/store/f2ld3hjbxnafi3jw299nq4mc94xpcmx6-android-tools-33.0.3p1/bin/avbtool*
d4357c43798d6aa6d5cea3b8f3ad39e54244a1105db8048dc673c44119835e1c  /nix/store/f2ld3hjbxnafi3jw299nq4mc94xpcmx6-android-tools-33.0.3p1/bin/avbtool
d4357c43798d6aa6d5cea3b8f3ad39e54244a1105db8048dc673c44119835e1c  /nix/store/f2ld3hjbxnafi3jw299nq4mc94xpcmx6-android-tools-33.0.3p1/bin/avbtool.py

[0]: https://github.com/nmeum/android-tools/releases/tag/33.0.3p1
[1]: 0652cba5f4

pkgs/tools/misc/android-tools/default.nix

@@ -35,10 +35,6 @@ stdenv.mkDerivation rec {
     export GOCACHE=$TMPDIR/go-cache
   '';
 
-  postInstall = ''
-    install -Dm755 ../vendor/avb/avbtool.py -t $out/bin
-  '';
-
   meta = with lib; {
     description = "Android SDK platform tools";
     longDescription = ''
@@ -53,7 +49,8 @@ stdenv.mkDerivation rec {
       - mke2fs.android (required by fastboot)
       - simg2img, img2simg, append2simg
       - lpdump, lpmake, lpadd, lpflash, lpunpack
-      - mkbootimg, unpack_bootimg, repack_bootimg
+      - mkbootimg, unpack_bootimg, repack_bootimg, avbtool
+      - mkdtboimg
     '';
     # https://developer.android.com/studio/command-line#tools-platform
     # https://developer.android.com/studio/releases/platform-tools