Improve process to run the Vendor HAL gTest suite.

This includes:
 - Adding a README to help users get started.
 - Improving the test script to avoid issue where
   device is not ready for the next ADB command
   after 'adb root'.

Bug: 271914366
Test: Manual test
       1. Reboot device.
       2. ./runtests.sh
       3. Tests are expected to execute.
Change-Id: I9f452384a7067708f617d1e932f06b2f5c7ab119
diff --git a/wifi/aidl/default/tests/runtests.sh b/wifi/aidl/default/tests/runtests.sh
index 1f53ab8..228c82b 100755
--- a/wifi/aidl/default/tests/runtests.sh
+++ b/wifi/aidl/default/tests/runtests.sh
@@ -20,7 +20,8 @@
 fi
 set -e
 
-$ANDROID_BUILD_TOP/build/soong/soong_ui.bash --make-mode android.hardware.wifi-service-tests
 adb root
+adb wait-for-device
+$ANDROID_BUILD_TOP/build/soong/soong_ui.bash --make-mode android.hardware.wifi-service-tests
 adb sync data
 adb shell /data/nativetest64/vendor/android.hardware.wifi-service-tests/android.hardware.wifi-service-tests