Propagate StartInputReason to attachNewInputLocked

As a preaparation to work on Bug 35079353, this CL makes it clear that
IMS#startInput()/IMS#restartInput() can be also be triggered by
IMMS#onSessionCreated().

By doing this, we can now assume that each
IMS#startInput()/IMS#restartInput() event has always one corresponding
StartInputReason. In a subsequent CL, we will pass this to the IME
process (and to the app process) for further traceability.

Test: no behavior change.
Bug: 35079353
Change-Id: I604e35b023d27b77126cd70fe068dc6b5edc2611