libfstab: Optimize out C++ object copy

* Edit / truncate string objects in-place, don't copy a temporary string
  object just for storing intermeidate results.
* Replace copy construct semantics with move semantics.
* Use range-based std::vector::insert() to move whole range.

Bug: 293695109
Test: CtsFsMgrTestCases
Change-Id: I5437303ba9900dbad3276a981413cba138f17157