Add a way to turn off unique_fd's operator int.

unique_fd's implicit conversion to int has led to tons of problems (see
all of the overloads for close, fdopen, fdopendir, etc.). Add a switch
that can turn it off, and reduce the ridiculous amount of work to fix up
callers by introducing a borrowed_fd type that can be constructed from
either int or unique_fd.

Test: treehugger
Change-Id: If77cf5cbcaddacdaec5919a15b3520fb68f51a62
diff --git a/adb/Android.bp b/adb/Android.bp
index 01e00dd..b87f4cb 100644
--- a/adb/Android.bp
+++ b/adb/Android.bp
@@ -26,6 +26,7 @@
         "-Wvla",
         "-DADB_HOST=1",         // overridden by adbd_defaults
         "-DALLOW_ADBD_ROOT=0",  // overridden by adbd_defaults
+        "-DANDROID_BASE_UNIQUE_FD_DISABLE_IMPLICIT_CONVERSION=1",
     ],
     cpp_std: "experimental",