DSPManager: Save and load presets from external storage

This patch adds a "Load preset" and "Save preset" option to the
menu of DSPManager, so they:
1) Survive a wipe
2) Can be swapped as you swap headphones, without having to re-do
   all your settings

Change-Id: I67ba7482404544ca99589830599c091ceb1f3b7c

Conflicts:
	res/menu/menu.xml
	src/com/bel/android/dspmanager/activity/DSPScreen.java