Refactor TCMS by centralizing the request handling logic

All the requests handling methods (e.g. onSuggestSelection, onClassifyText)
are  implemented in a very similar fashion. Pull out the logic to the
handleRequest() method.

This is also for the preparation of the upcoming CL that will introduce
the default System TC.

Test: mts run mts-extservices
BUG: 148049185

Change-Id: I8fe74bf74b709cc7d204528c483ba285b25969e1