Fix (still-disabled) sharesheet "token flow."

Per b/132321687, we need to leave the system-side share
activity open (not call finish()) until the SysUI-side
chooser finishes, or else we'll invalidate the start-
activity token that the SysUI (delegate) chooser needs
when it launches the target activity.

Note that the delegated flow is still disabled by the
flag in DisplayResolveInfo.

Bug: 132321687,199743918
Test: Reproduced crash condition & confirmed fix manually.
Change-Id: Ic467dfe4eb417ca846c0ecf2e8a0067bd5275e9f