sign_apex.py respects hash algorithm

When sign_apex.py re-signs an apex, the hash algorithm is extracted and
it is used when re-packaging the file.

Bug: 155771970
Test: releasetools_py3_test doesn't regress
(some tests were failed, but they failed even without this change:
test_ApexApkSigner_apkKeyNotPresent (test_apex_utils.ApexUtilsTest) ... ERROR
test_ApexApkSigner_noApkPresent (test_apex_utils.ApexUtilsTest) ... ERROR
test_ApexApkSigner_noAssetDir (test_apex_utils.ApexUtilsTest) ... ERROR
test_ApexApkSigner_signApk (test_apex_utils.ApexUtilsTest) ... ERROR)

Change-Id: I996a33e0208d3dd3a04a31b8dafb27ef995297f2