releasetools: Make _GetPropertyFilesString public

ota_from_target_files.py:

Rename PropertyFiles._GetPropertyFilesString to
PropertyFiles.GetPropertyFilesString.

Currently only PropertyFiles.Compute and
PropertyFiles.Finalize gives access to _GetPropertyFilesString.
But Compute force sets reserve_space=True,
and Finalize requires reserved_length.

_GetPropertyFilesString is useful method that can
be used outside releasetools.

One of the usage is in bootable/recovery/updater_sample/.

Test: python -m unittest test_ota_from_target_files
Change-Id: I2cc44ec46a0e68ba071531b003af8cdbfe90e588
Signed-off-by: Zhomart Mukhamejanov <zhomart@google.com>