Bug 1163082 - Part 2: Extract Android distribution from packaged assets rather than APK root. r=rnewman

This reads from "assets/distribution/**" in the APK and writes to
"distribution/**" in the data directory.  That output is the same, but
the input used to read from "distribution/**", which is not really
supported by modern build tooling (Gradle), which doesn't allow to
write files directly into the APK root.

I manually tested this without issue.  I see no way to add meaningful
tests to our current Robocop test suite; the long term testing
approach is to develop a new test for this functionality and only run
it against the "distribution" build type that was added in Bug
1163080.  However, that's a larger project than I have time for now.

mobile/android/base/java/org/mozilla/gecko/distribution/Distribution.java

@@ -643,7 +643,7 @@ public class Distribution {
     private boolean copyAndCheckAPKDistribution() {
         try {
             // First, try copying distribution files out of the APK.
-            if (copyFiles()) {
+            if (copyFilesFromPackagedAssets()) {
                 // We always copy to the data dir, and we only copy files from
                 // a 'distribution' subdirectory. Now determine our actual distribution directory.
                 return checkDataDistribution();
@@ -718,13 +718,16 @@ public class Distribution {
     }
 
     /**
-     * Copies the /distribution folder out of the APK and into the app's data directory.
+     * Copies the /assets/distribution folder out of the APK and into the app's data directory.
      * Returns true if distribution files were found and copied.
      */
-    private boolean copyFiles() throws IOException {
+    private boolean copyFilesFromPackagedAssets() throws IOException {
         final File applicationPackage = new File(packagePath);
         final ZipFile zip = new ZipFile(applicationPackage);
 
+        final String assetsPrefix = "assets/";
+        final String fullPrefix = assetsPrefix + DISTRIBUTION_PATH;
+
         boolean distributionSet = false;
         try {
             final byte[] buffer = new byte[1024];
@@ -739,11 +742,14 @@ public class Distribution {
                     continue;
                 }
 
-                if (!name.startsWith(DISTRIBUTION_PATH)) {
+                // Read from "assets/distribution/**".
+                if (!name.startsWith(fullPrefix)) {
                     continue;
                 }
 
-                final File outFile = getDataFile(name);
+                // Write to "distribution/**".
+                final String nameWithoutPrefix = name.substring(assetsPrefix.length());
+                final File outFile = getDataFile(nameWithoutPrefix);
                 if (outFile == null) {
                     continue;
                 }