releasetools: Fix an issue in common.GetSparseImage

Fix incorrect report "Failed to find the ZIP entry" error when
generate OTA package.

This happens when we have a file in ROOT which name contains 'system'
such as init.system_init.rc, and init.system_init.rc will be rename to
init.SYSTEM_init.rc incorrectly, so we failed to find the entry in ZIP.

Test: manual test
Change-Id: I97359e513aaca7521fe9c035f6a4264a2053b86d
Signed-off-by: wangshumin <wangshumin@xiaomi.com>