Android: Remove direct use of Action strings, use dynamic lookup

The Bluetooth for Android port uses a lot of actions which are defined as static final String. Currently the strings are directly used. There is no guarantee that strings don't change. Therefore the static fields should rather be used.

This requires some caching to avoid too much performance drain.

This was highlighted by:
https://codereview.qt-project.org/#patch,sidebyside,77809,4,src/bluetooth/android/devicediscoverybroadcastreceiver.cpp