Add onUnbind from OverviewProxyService, and destroy taskbar

When unbound, destroy taskbar immediately instead of waiting for
TIS#onDestroy. And then send the reply back to OverviewProxyService
so it can bind to a new TIS.

Fixes: 375315062
Test: Switch users 10 times, no crash (also check logs)
Flag: EXEMPT bug fix

Change-Id: I737c47c7df87e11608672a4e16eb3e9fd645e487