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
1 file changed