Ziparchive: Enable -Wconversion

Enable -Wconversion (but not -Wsign-conversion). Fix up code. Handle
some actual error cases:

* too many files
* files too large

Bug: 130039052
Test: atest ziparchive-tests
Change-Id: I632af317b9e65dbc728851efefd0d11a2b5c29b9
diff --git a/Android.bp b/Android.bp
index bc1543b..858c0bb 100644
--- a/Android.bp
+++ b/Android.bp
@@ -26,6 +26,8 @@
         // Incorrectly warns when C++11 empty brace {} initializer is used.
         // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61489
         "-Wno-missing-field-initializers",
+        "-Wconversion",
+        "-Wno-sign-conversion",
     ],
 
     // Enable -Wold-style-cast only for non-Windows targets.  _islower_l,