surfaceflinger: clear HWC layers in Layer::onRemoved

Explicitly destroy HWC layers in Layer::onRemoved rather than doing
that implicitly in the destructor.  The layer may be destructed by
any thread that holds a sp<Layer>, but only the main thread should
talk to the composer.

Bug: 38151478
Bug: 37978067
Test: manual
Change-Id: If6fcfaa5a70d247b751f2b4d9aec9eca890e3aa5