Auto-detect minor version number for partial updates

Partial updates require minor version 7. Instead of requiring the caller
to pass in --minor_version=7, let delta_generator auto detect the minor
version number.

Test: th
Bug: 293313353
Change-Id: Iaae7616ab25aca713ede2f650061540ff8014218
diff --git a/payload_generator/generate_delta_main.cc b/payload_generator/generate_delta_main.cc
index 8fa86f0..5f0c9d2 100644
--- a/payload_generator/generate_delta_main.cc
+++ b/payload_generator/generate_delta_main.cc
@@ -739,6 +739,10 @@
     if (payload_config.is_delta) {
       LOG(FATAL) << "Minor version is required for delta update!";
       return 1;
+    } else if (FLAGS_is_partial_update) {
+      payload_config.version.minor = kPartialUpdateMinorPayloadVersion;
+      LOG(INFO) << "Using minor_version=" << payload_config.version.minor
+                << " for partial updates";
     } else {
       payload_config.version.minor = kFullPayloadMinorVersion;
       LOG(INFO) << "Using non-delta minor_version="