Create a function that can generate ApexInfo using target-files

If an OTA contains compressed APEX inside it, then the device will need
to allocate space on /data partition for their decompression. In order
to calculate how much space the OTA process needs to allocate, the
process needs more information about the APEX contained inside the OTA.

In this CL, we are adding functionality to the OTA generation script
that allows us to gather information about the APEX stored inside the
target-file zip. However, we did not integrate the new functionality
with the ota_from_target_files.py scrip yet. That will be done on follow
up CL.

Bug: 172911822
Test: atest releasetools_py3_test
Change-Id: I2ac42018f628c2c21527b3e086be1f4e7e7247ad