Instead of using your own flag, you can use the flag set automatically by ADT, like this:
final static int appFlags = context.getApplicationInfo().flags;
final static boolean isDebug = (appFlags & ApplicationInfo.FLAG_DEBUGGABLE) != 0
The FLAG_DEBUGGABLE bit is automatically set to true or false, depending on the “debuggable” attribute of the application (set in AndroidManifest.xml). The latest version of ADT (version 8) automatically sets this attribute for you when not exporting a signed package.
Thus, you don’t have to remember setting / resetting your own custom flag.
You can read more in this thread.