Move parts of roots.cpp from librecovery

Move some mounting functionalities from librecovery so that
other modules can also mount logical partitions correctly.

- Also, refactor to use C++ Fstab struct.
- Also, fixed CreateLogicalPartitions() to take full path of
super device.

Test: run recovery tests
Test: vintf_object_recovery_test

Bug: 118634720
Bug: 113182233
Bug: 62292478
Change-Id: Ie02c3a941ed801fdae5818099faf5f3863444296