envsetup.sh: Use prebuilt emulator binaries if available.

This patch changes the setpaths() function in envsetup.sh to probe
for prebuilts/android-emulator/<host>/, and prepend it to PATH if
it exists.

See https://googleplex-android-review.googlesource.com/#/c/459741/
for a related patch that adds the binaries.

BUG=13747402

Change-Id: I30794ea52f6dfc58908e6271f2c0da8e2f0b68e8

envsetup.sh

@@ -189,6 +189,24 @@ function setpaths()
     unset ANDROID_HOST_OUT
     export ANDROID_HOST_OUT=$(get_abs_build_var HOST_OUT)
 
+    # If prebuilts/android-emulator/<system>/ exists, prepend it to our PATH
+    # to ensure that the corresponding 'emulator' binaries are used.
+    case $(uname -s) in
+        Darwin)
+            ANDROID_EMULATOR_PREBUILTS=$T/prebuilts/android-emulator/darwin-x86_64
+            ;;
+        Linux)
+            ANDROID_EMULATOR_PREBUILTS=$T/prebuilts/android-emulator/linux-x86_64
+            ;;
+        *)
+            ANDROID_EMULATOR_PREBUILTS=
+            ;;
+    esac
+    if [ -n "$ANDROID_EMULATOR_PREBUILTS" -a -d "$ANDROID_EMULATOR_PREBUILTS" ]; then
+        PATH=$ANDROID_EMULATOR_PREBUILTS:$PATH
+        export ANDROID_EMULATOR_PREBUILTS
+    fi
+
     # needed for building linux on MacOS
     # TODO: fix the path
     #export HOST_EXTRACFLAGS="-I "$T/system/kernel_headers/host_include