AudioAttributes: fix regression with setLegacyStreamType in Builder

A regression was introduced by commit @6d69bde3 which caused a new
  Builder instance to be created when using setLegacyStreamType(),
  instead of returning the current one.

Bug: 130274354
Test: atest android.media.cts.AudioAttributesTest#testSetLegacyStreamOnBuilder
Change-Id: Ib66a0beb5f0f4489a6003cb9be6b8313b3318eac