Switch imgdiff to use libz_stable

libz contain platform dependent optimization flags, and sometimes that
cause reconstruction of blobs to fail. Use libz_stable instead

Bug: 177076632
Test: treehugger

Change-Id: I3a8c1591672537d1c754b2bc5b26f939dd80ed47
diff --git a/tests/Android.bp b/tests/Android.bp
index 19f2a6c..d2179e4 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -31,7 +31,6 @@
         "libpng",
         "libprocessgroup",
         "libselinux",
-        "libz",
         "libziparchive",
     ],
 
@@ -65,7 +64,7 @@
     "libbase",
     "libbrotli",
     "libbz",
-    "libz",
+    "libz_stable",
     "libziparchive",
 ]