Some cleanups to recovery.

- Remove the duplicate gCurrentUI variable in recovery.cpp;
- Refactor the load/save of locale functions;
- Clean up ui_print() to get rid of 256-byte buffer limit;
- Declare ui in common.h;
- Move the typedef of Volume into roots.h.

Test: Build and boot into recovery image.

Change-Id: Ia28c116858ca754133127a5ff9c722af67ad55b7