Remove Android 10 testing warning from readme and update buildtool

* Removes warning we had given to users last release about Android 10 (Q) testing
* Reverts the patch where we were checking out main branch of Arm NN instead of
  latest release branch in Buildtool
* Updated Android API level in the bianry table in readme

Signed-off-by: Nikhil Raj <nikhil.raj@arm.com>
Change-Id: I1e497b7d0ecf8fa7d9dd7262ca740052c0db444e