Fix DSUEndtoEndTest to work with new system image format

As getDeviceImageFile() may return the image either as a zip file or uncompressed as a dir with the same name, make this change to support it.

Bug: 368129183
Change-Id: I6a0ba9e1913709886d50e17b39b31910832c195e
Test: https://android-build.corp.google.com/builds/abtd/run/L64600030006540096
1 file changed