Fix logic to figure out what /data fstab entry was mounted

Now the logic is handled by following API:
GetMountedEntryForUserdata(Fstab* fstab).

Behind the scenes it does the following:
1. Reads /proc/mounts and gets block device /data is actually mounted on.
2. In case of it's a dm-device, recursively goes into it's slaves until
finds the underlying block device.
3. Optimistically assumes that corresponding block device is a symlink
and tries to read it.
4. Reads all the entries corresponding to /data from fstab.
5. For each of them, optimistically tries to readlink block_device.
6. If it matches the resolved block devices, we found our fstab entry!

Also added a test to CtsFsMgrTestCases asserting that /data was mounted
from one of entries in default fstab.

Test: on blueline & taimen, with & without checkpointing:
Test: atest CtsFsMgrTestCases
Test: adb reboot userspace

Bug: 135984674
Change-Id: Ic70daeeb18096c7b134004334cc674dacc6e36f3