releasetools: Address two issues with system_root_image file map.

With system_root_image, e2fsdroid writes file map with extra leading
slashes in filenames (e.g. "//system/framework/am.jar"). This breaks the
detection of files with incomplete ranges, and thus fails the patch
generation. This CL addresses the issue by stripping out leading
slashes.

Additionally, non-/system files (e.g "//sbin/charger") are not packed
under SYSTEM/ in a target_files.zip, despite being part of system.img.
We need to look for these files under ROOT/ instead.

This CL also asserts the availability of all files listed on a file map,
to avoid silently missing other edge cases.

Bug: 80380658
Test: python -m unittest test_common
Test: Successfully generated an incremental for a target using
      system_root_image that was previously failing.
Change-Id: I62a2460e882f3930e99add4d2b44291edf7a51a0