boot_control_copy: Fix getCurrentSlot().

The getCurrentSlot() method wasn't returning the right value if
setActiveBootSlot() was called with another slot since last reboot.

Instead of peeking in the misc partition, just look at the dev_t for
/system and compare it with the dev_t for system_a and system_b.

Bug: 24201805
Change-Id: I601ab5e355e0f423637e6867d0afda97f68b57fd
1 file changed