Move partition writer to a separate file

Test: treehugger && serve an OTA update
Change-Id: I803692110841ce6d2207555ac7a682e9f989363d
diff --git a/Android.bp b/Android.bp
index acd3633..8e9ec17 100644
--- a/Android.bp
+++ b/Android.bp
@@ -178,6 +178,7 @@
         "payload_consumer/payload_constants.cc",
         "payload_consumer/payload_metadata.cc",
         "payload_consumer/payload_verifier.cc",
+        "payload_consumer/partition_writer.cc",
         "payload_consumer/postinstall_runner_action.cc",
         "payload_consumer/verity_writer_android.cc",
         "payload_consumer/xz_extent_writer.cc",