AudioService: getMasterStreamType -> RING on tablets.

Since all service-internal checks use stream alias, and the alias
for NOTIFICATION is RING, even on tablets.

This is a followup to ag/506532.

Ensure that #getMasterStreamType always follows the stream_system
alias, and update the system alias for tablets per the new policy.
Update the associate javadoc comment in AudioManager to make this
clear.

Note: there is now no difference in the alias maps between phones
and tablets.

Also fix a condition affected by this in VolumePanel.

Bug:16626879
Change-Id: I2700a48d2a3e7703607f771a1ab7f325596fd789