releasetools: Clean up AppendVBMetaArgsForPartition().

Drop the parameter of public_key_dir. It's not necessary to stage all
the extracted public keys into the same temporary dir.

Redirect the stderr output to STDOUT while calling avbtool, so that it
can dump the error message accordingly.

Also remove the check for having empty img_path arg. It doesn't look
like a clean logic to allow the function to be called with invalid /
empty arg. The only caller (i.e. AddVBMeta) already asserts img_path
being valid as well.

Test: python -m unittest test_add_img_to_target_files
Test: `m dist` with aosp_walleye-userdebug
Change-Id: Id58c5ae780ac8a22661ffea629144d4836839175