Allow override of default symbol location for native heap library resolution

Default location of the symbols is $ANDROID_PRODUCT_OUT/symbols.
Setting the env variable ANDROID_SYMBOLS allows you to override this location
so that DDMS goes look for the symbol in a different location when
resolving native heap addresses.

tools/ddms/libs/ddmuilib/src/com/android/ddmuilib/Addr2Line.java

@@ -133,7 +133,10 @@ public class Addr2Line {
         // to synchronize this code.
 
         // get the output directory.
-        String symbols = DdmUiPreferences.getSymbolDirectory();
+        String symbols = System.getenv("ANDROID_SYMBOLS");
+        if (symbols == null) {
+            symbols = DdmUiPreferences.getSymbolDirectory();
+        }
 
         // build the command line
         String[] command = new String[5];