Make copy_media.sh POSIX compatible

String comparision '==' is specific to bash, and not compatible with POSIX
shells like dash, which uses only '=' to test string equality

bug:25727833
Change-Id: I160351d272e018d7eee7d872e6527b009667bec3
diff --git a/tools/cts-media/copy_media.sh b/tools/cts-media/copy_media.sh
index cf0d099..cfec807 100755
--- a/tools/cts-media/copy_media.sh
+++ b/tools/cts-media/copy_media.sh
@@ -17,15 +17,15 @@
 max_resolution=3
 if [ $# -eq 0 ]; then
   echo "assuming default resolution"
-elif [ "$1" == "-s" ]; then
+elif [ "$1" = "-s" ]; then
   adb_options=""$1" "$2""
-elif [ "$1" == "720x480" ]; then
+elif [ "$1" = "720x480" ]; then
   max_resolution=1
-elif [ "$1" == "1280x720" ]; then
+elif [ "$1" = "1280x720" ]; then
   max_resolution=2
-elif [ "$1" == "1920x1080" ]; then
+elif [ "$1" = "1920x1080" ]; then
   max_resolution=3
-elif [ "$1" == "all" ]; then
+elif [ "$1" = "all" ]; then
   max_resolution=3
 else
   echo "Usage: copy_media.sh [720x480|1280x720|1920x1080] [-s serial]"