Enable root before calling INativeDevice#pushString

1. We need root to push files to folders like
   /data/user_de or /data/misc_ce successfully.

2. Disable root at the end of each helper method so
   we don't accidentally run the following code as root.

Bug: 170804667
Test: atest StagedRollbackTest
Change-Id: Icfa8054eafa9230a1893bde33b8f0005f8b6c69d