get_mounted_entry_for_userdata: Realpath block devices from fstabs

Use realpath as a canonical representation of block devices. This makes
it easier to reason about block devices. This also fixes a bug, in which
fs_mgr_get_mounted_entry_for_userdata didn't properly work on devices
that don't support metadata encryption.

Test: atest CtsFsMgrTestCases
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 153363818
Change-Id: I139c2be46336a632bbaee86667019c075d7de814