Add support for compressed snapshot merges in fastboot.

SnapshotManager::New() is now preferred in recovery. Previously we used
NewForFirstStageMount(), which is technically incorrect as that enables
code paths specifically for first-stage init.

We also explicitly label the snapuserd context, since rootfs in recovery
has unlabelled files.

Finally, we add a timeout to internal calls to
CreateSnapshotsAndLogicalPartitions. Without this, WaitForDevice() calls
will terminate immediately, which breaks VABC given the more complex
device stacking that is created.

Bug: 168258606
Test: fastboot snapshot-update merge
Change-Id: I3a663b95c0b1eabaf14e6fde409c6902653c3c5e