fs_mgr: convert parse_flags over to C++

Fixes a bug where verify= and a path is not respected.
Keeps the default value of -1 for swap_prio, zram_size, and the vold
managed partition number in the case of not being able to parse their
values.

Test: fs_mgr unit tests, boot
Change-Id: I41bd92a7a2f9165d62134704cb7a1209d76a2071