Fix broken test by increasing Thread#join timeout

The timeout was added in ag/19606284 but it seems 1ms wasn't enough for
the work to complete so the test as almost 100% flaky.

Test: atest UserBackupManagerServiceTest
Fixes: 276437270
Fixes: 309507814
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bf8a676082dc24a3397c412c9b8b8e68da8f18d1)
Merged-In: Ie8ff85d8fc52ae9e9d80778131afc0bf8a8eedd6
Change-Id: Ie8ff85d8fc52ae9e9d80778131afc0bf8a8eedd6

NOTE FOR REVIEWERS - errors occurred while applying the patch.
PLEASE REVIEW CAREFULLY.
Errors:
Error applying patch in services/tests/mockingservicestests/src/com/android/server/backup/UserBackupManagerServiceTest.java, hunk HunkHeader[84,7->84,7]: Hunk cannot be applied

Original patch:
 From bf8a6760 Mon Sep 17 00:00:00 2001
From: Sarp Misoglu <sarpm@google.com>
Date: Wed, 05 Apr 2023 14:25:06 +0100
Subject: [PATCH] Fix broken test by increasing Thread#join timeout

The timeout was added in ag/19606284 but it seems 1ms wasn't enough for
the work to complete so the test as almost 100% flaky.

Test: atest UserBackupManagerServiceTest
Fixes: 276437270

Change-Id: Ie8ff85d8fc52ae9e9d80778131afc0bf8a8eedd6
---