Preserve the SDK images between upgrades

The `android` tool downloads the SDK images into its prefix. Which means you have to move them manually to the new android-sdk keg when you upgrade. Which sucks. So we'll store them in the var/lib/android-sdk directory because that makes the most sense based on my interpretation of the POSIX filesystem hierarchy documentation.

We still have to symlink the var directories back into the keg, because the SDK is stupid.

For discussion: whether we should use var in this manner more.

Formula/android-sdk.rb

@@ -6,31 +6,35 @@ class AndroidSdk <Formula
   md5 '0f330ed3ebb36786faf6dc72b8acf819'
   version 'r7'
 
-  skip_clean 'add-ons'
-  skip_clean 'platforms'
-  skip_clean 'temp'
+  VAR_DIRS = %w[platforms docs samples temp add-ons]
+
+  skip_clean VAR_DIRS
 
   def install
-    mkdir %w[temp docs] << bin
+    mkdir bin
 
-    mv 'SDK Readme.txt', 'README'
-    prefix.install Dir['*']
+    mv 'SDK Readme.txt', prefix/'README'
+    mv 'tools', prefix
 
     %w[adb android apkbuilder ddms dmtracedump draw9patch emulator
            hierarchyviewer hprof-conv layoutopt mksdcard traceview
            zipalign].each do |tool|
-      (bin+tool).make_link(prefix+'tools'+tool)
+      (bin/tool).make_link(prefix/'tools'/tool)
+    end
+
+    # this is data that should be preserved across upgrades, but the Android
+    # SDK isn't too smart, so we still have to symlink it back into its tree.
+    VAR_DIRS.each do |d|
+      dst = prefix/d
+      src = var/'lib/android-sdk'/d
+      src.mkpath unless src.directory?
+      dst.make_relative_symlink src
     end
   end
 
-  def caveats; <<-EOS.undent
-    We agreed to the Android SDK License Agreement for you when we downloaded the
-    SDK. If this is unacceptable you should uninstall. You can read the license
-    at: http://developer.android.com/sdk/terms.html
-
-    Please add this line to your .bash_profile:
-
-        export ANDROID_SDK_ROOT=#{prefix}
+  def caveats; <<-EOS
+    We put the useful tools in the PATH. Like the `android` tool. You probably
+    want to run that now.
     EOS
   end
 end