Pull backup password logic out of BackupManagerService

This makes it easier to follow what's going on. Also makes it easy for us to
add unit tests for this component.

Bug: 36850431
Test: adb shell am instrument -w -e package com.android.server.backup com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Ifb85f1573bcfb7da26febed35d49454614b5d975