nix: Add android-tools to shell.nix

Otherwise the `adb reverse` command in the instructions
isn't available.

(I had an equivalent of android-tools as part of a draft branch
to get the Android emulator from the command line, too, and I
guess didn't test this part of the instructions on this version
of the shell.nix file.)

Author: gnprice

shell.nix

@@ -36,6 +36,7 @@ mkShell {
 
     jdk11
     android-studio
+    android-tools
 
     # Used by various `tools/` scripts:
     git