idsig: move apksigv4.rs into libidsig

The library is the place where everything about the handling of
idsig format is. Move apksiv4.rs from apkdmverity to the library.

The behavior remains the same.

Bug: 193504400
Test: m apkdmverity libidsig
Change-Id: I7994fee83f5a8fcd7e8988ceeb9bbfe7a47a684f
diff --git a/apkdmverity/Android.bp b/apkdmverity/Android.bp
index 9b53a47..df46324 100644
--- a/apkdmverity/Android.bp
+++ b/apkdmverity/Android.bp
@@ -13,6 +13,7 @@
         "libbitflags",
         "libclap",
         "libdata_model",
+        "libidsig",
         "liblibc",
         "libnix",
         "libnum_traits",