services: MountService: Refactor MountService for vold2

Squash of the following:
services: MountService: Rework the way volume states are handled
MountService: Add new API for directly getting volume state via a mount point
Environment: Switch from using system property for external storage state.
MountService: Add support for UMS
MountService: Fix a few bugs
services: MountService: Add support for mount-on-insertion
services: MountService: Add some debugging around UMS
services: MountService: Fix some UMS bugs and clean-up startup mount code

Signed-off-by: San Mehat <san@google.com>