Do not write backup enabled status to file on init

 When backup service is brought up, we read the enabled status
from a file and store it in a class variable in
UserBackupManagerService. However, we also write the value we just read
back into the file. This is an unncessary operation and it makes the
code more fragile: if we read the wrong value due to some I/O error,
we'll then write it back making the error permanent.

More details here: https://docs.google.com/document/d/1GtR2HYMiSJuy0wuxBg8n_zkEJGl3C5apcvpvKheNa_A/edit

Bug: 149084461
Test: 1. "bmgr enable true"; verify "bmgr enabled" returns true; reboot; verify "bmgr enabled" returns true
      2. "bmgr enable false"; verify "bmgr enabled" returns false; reboot; verify "bmgr enabled" returns false
      3. atest RunBackupFrameworksServicesRoboTests
      4. atest FrameworksServicesTests:UserBackupManagerServiceTest
Change-Id: If213a6b1de0de51c9337138be347c304b44689d1