Update adb backup/restore confirmation activity on new intent

Now, if adb backup/restore is currently active and the activity is
passed a new intent, the activity instance would be updated with the new intent.
Same for if the activity is relaunched.

Before, the activity would ignore intent changes and use old tokens and
views, leading to backing up/restoring wrong files.

Bug: 65723308
Test: adb backup -f old.ab -all; ctrl-c; adb backup -f new.ab -all;
gts-tradefed run commandAndExit gts-dev -m GtsBackupHostTestCases
Change-Id: Ifa11efecf82a682a578dab7e5795328ab7d0c054