Update eligibility rules for adb backup

1. Adb backup is enabled by default for "android" package that corresponds to SystemBackupAgent (no change from current behavior)
2. System and privileged apps can use android.backup.ALLOW_ADB_BACKUP manifest property to enable / disable adb backup. Disabled by default.
3. Other apps can only use adb backup when running in debuggable mode.

Bug: 171032338
Test: 1. atest BackupEligibilityRulesTest
      2.1. Run adb backup / restore for SystemBackupAgent and verify
           success (running in system_server)
      2.2. Run adb backup / restore for NexusLauncher with
           "allowAdbBackup=true" and verify success (privileged app)
      2.3. Run adb backup / restore for a non-privileged debuggable app
           and verify success.
Change-Id: Ifefe6d888377d3ac9482928b27c86b2e562ad8fa