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