[BREAKING CHANGE] ButtonWidget: Don't default 'target' to '_blank'

target=_blank has its use cases, but it definitely shouldn't be the
default behavior.

Bug: T75452
Change-Id: I88b7bc1b562bca2a63b611b1d33dd9d534297e68
Merge "[BREAKING CHANGE] ButtonWidget: Don't default 'target' to '_blank'"