Always set android debuggable=false unless disabled by grunt config

In Phonegap 4.3.x, the release apk is created with debuggable=true regardless of whether a debug certificate was used. In Phonegap 4.4.x, (I believe) this has been corrected.

To ensure we get the intended behavior regardless of Phonegap version, grunt-phonegap will post-process AndroidManifest.xml and set `debuggable=false` **UNLESS** `phonegap.config.debuggable` is set to true, in
which case that value will be forced instead.

To generate a debuggable APK, you should now use the new task `phonegap:debug:android` (see #83).

This change should resolve #61.