fstab: allow mounting other slot

Adding a new fs_mgr flag 'slotselect_other' to mount _b when _a is
active, or vice versa.

Bug: 113182233
Bug: 112103720
Test: `mount_all fstab.test` with one line using `slotselect_other`.

Change-Id: I96c63141df7722dc30ca9817b0b154e493b9eef9