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

tools/releasetools/test_utils.py

@@ -55,6 +55,11 @@ def get_testdata_dir():
   current_dir = os.path.dirname(os.path.realpath(__file__))
   return os.path.join(current_dir, 'testdata')
 
+def get_current_dir():
+  """Returns the current dir, relative to the script dir."""
+  # The script dir is the one we want, which could be different from pwd.
+  current_dir = os.path.dirname(os.path.realpath(__file__))
+  return current_dir
 
 def get_search_path():
   """Returns the search path that has 'framework/signapk.jar' under."""