androidStudioPackages.beta: Rename the binary according to the channel

The old name "android-studio-preview" was a bit misleading while
"android-studio-beta" should clearly reflect that this is from the beta
channel. I hope that this does not break any workflows but since
"android-studio-preview" was most likely not called from any scripts the
risk should be low (also: most people probably use the stable version
anyway).