Fix incremental OTA generation failures

source_file might be a directory, use the _ReadApexInfo() helper
function to read it instead of go straight to zipfiles.

Test: build incremental OTA for pixel
Bug: 279142680
Bug: 227848550
Bug: 277028723

Change-Id: Ia0041e2c72c1a57123519c970f6e734194a81cd2