Add debug build type and enable minification

This branch adds a debug build type so that we can start to separate debug builds from release builds. ProGuard minification is enabled for debug and release builds so that we catch any problems early.

Note that launching the app on an API 21-23 device or emulator from Android Studio doesn't work with minification enabled. Two possible workarounds are to build and install the APK from the command line, or launch the app on two devices/emulators, one of which is outside the API range 21-23.

Closes #173 (closed)

Test instructions:

• With this MR, the mailbox app installs with a different ID onto your Android devices. If you have the Mailbox app already installed, installing from this branch will not replace that existing app but install side by side. The old launcher on the device home screen if any still goes to the old app. It can be confusing to find the correct app, so it's probably best to uninstall any prior versions before installing this one. (The new app will have a different launcher name though: Briar Mailbox Debug.
• Test everything is running fine with this app. This MR introduces changes to the build system, so things could break at unexpected places within the UI and workflows. Best is to go through the major usage scenarios like linking, unlinking and get reassured that the app is still overall working as expected.