AudioService: suppress superfluous volume changed intents

Do not broadcast VOLUME_CHANGED_ACTION intent if the externally visible
volume index does not change: this can happen on aliased streams when
the internal volume index changes but the externally visible index
(divided by 10) does not change due to rounding.

Bug: 156835211
Test: verify intent broacasts in activity manager manually.
Change-Id: I98f25d175ada39fe1d2bc4f7d008f20a5d29ef16