releasetools: Group the option descriptions in ota_from_target_files.py.

This CL separates the options into three groups (excluding the global
options provided via common.py).
 - Non-A/B OTA specific options;
 - A/B OTA specific options;
 - Common options that apply to both.

It mostly reshuffles the lines, with minor change to "--verify" that
removes the obselete remounting behavior (which doesn't apply to
block-based OTA).

Hopefully this makes the expected behavior of some options less
confusing.

Test: `build/make/tools/releasetools/ota_from_target_files.py`
Change-Id: I194ea52c4f7d6a3c7f34531abbcf3fdc7b7f4fa8