Use libbinderthreadstateutils.

Clients needing to differentiate between system/vendor used to be able
to use isServingCall. However, this was expensive to implement. This
alternative approach instead requires users of this API to use
libbinderthreadstateutils which does a slighty different operation.

Bug: 148692216
Test: boot
Change-Id: I2c926107e43a47bd4e20e08f16a293e707ab03f7