Extend the bootcontrol HAL to get the active slot

The existing HAL has a setter, but no getter for the active slot for
next boot. The information is useful. For example, clients may need
to figure out which build the device will boot into.

In the resume on reboot case, we want to put the vbmeta digest
together with an encrypted key. And the vbmeta digest is associated
with a set of images (depending on which slot to boot into).

Bug: 173808057
Test: run vts test on cuttlefish
Change-Id: Ia8578926c781eb8e006589f45ab35a95dfe4f9a0