This CL adds the `create` and `write_into` methods to `V4Signature` each
of which is used to construct V4Signature from an apk and to write it
into a file.
Next step will be modifying the virtualization service to create an
idsig file for a given apk, thus eliminating the need to provide the
idsig file separately.
Bug: 193504400
Test: m libidsig libidsig.test
Change-Id: I2be60fbb6ec40af12297e20b112318a032dd78f9
Jiyong Park