Allow KeyChain.bindAsUser() to be called on the main thread

KeyChain.bindAsUser() couldn't be called on the main thread because
it was using the main thread to handle service connection callback.
Add an overload of KeyChain.bindAsUser() that accepts an alternative
handler to process the connection callback, which makes it possible
to call KeyChain from the main UI thread directly.

Bug: 165641221
Test: atest KeyChainTests
Test: m RunKeyChainRoboTests
Change-Id: I4290bccf5ae04de0d84c7091729e86704b937295