libsnapshot: add GetCurrentSlot

Factor out obscure logic that reads boot indicator path.
GetCurrentSlot() returns three states:
- None (read failure)
- Old (before reboot)
- New (after reboot)
Use these "logical" slot values for simpler code.

Test: libsnapshot_test
Change-Id: I1fa2ce4916c5a1652d25682ec1f11e101c858822