Iterate over vector in pilferPointersLocked

No behaviour change in this CL.
We are currently returning early from the function where there isn't
exactly one device active.

To future-proof this code, iterate over the entire vector of devices,
knowing that currently, only 1 iteration will occur.
This will simplify the future CL for multi-device support.

Bug: 211379801
Test: atest inputflinger_tests
Change-Id: Ie58ecde47f6dccd4cb45e1d8264af59376c27683