Improve magic space handling.

Split symbols into:
- Symbols that should swap themselves with a magic space.
- Symbols that should remove the magic space.
- Symbols that should transform the magic space into a real space.

Also, have symbols inserted from the suggestion strip always consider
the space before them to be magic.

Also, do minor refactoring to remove useless variables or make clearer
how to carry some symbol-related information.

Bug: 4391347
Change-Id: Ie2308a62c8fef55e3610897c0f58943acb7aa8c9