Detect downgrade by checking build time for all partitions

Checking ro.build.date.utc to detect a downgrade is not enough in the
case where several target files are merged, as timestamps can differ
for each partition.

The solution is to check the build timestamp for each partition and
ro.build.date.utc.

With this change ota generation will be aborted during creation
instead of rejected when installing on device.

This also fixes the --override_timestamp so it is possible to generate
a package where timestamps are reversed, as was added in:
3e6161a3b3.

Issue: 315913966
Test: Manual, confirm that ota generation is aborted when timestamp
  on post vendor partitions is newer than on pre.
  Ota created successfully with --override_timestamp parameter and
  installation successful
Test: atest --host releasetools_test
Change-Id: I275e67a3840f4ef2263381c253231068e72f47d2

tools/releasetools/ota_from_target_files.py

@@ -1038,7 +1038,11 @@ def GenerateAbOtaPackage(target_file, output_file, source_file=None):
   partition_timestamps_flags = []
   # Enforce a max timestamp this payload can be applied on top of.
   if OPTIONS.downgrade:
-    max_timestamp = source_info.GetBuildProp("ro.build.date.utc")
+    # When generating ota between merged target-files, partition build date can
+    # decrease in target, at the same time as ro.build.date.utc increases,
+    # so always pick largest value.
+    max_timestamp = max(source_info.GetBuildProp("ro.build.date.utc"),
+        str(metadata.postcondition.timestamp))
     partition_timestamps_flags = GeneratePartitionTimestampFlagsDowngrade(
         metadata.precondition.partition_state,
         metadata.postcondition.partition_state