SyncSM10: explicitly keep TetheringUtils jni methods

Found R8 accidentally remove TetheringUtils jni methods while doing
some refactor. If any of jni java methods is removed by R8, devices
would fail to boot because of jni registering failure. Explicitly
adding progurad rule to make sure R8 never remove it.

Test: boot, atest CtsTetheringTest

Change-Id: I091a2f094d01f683fb454ed4624be35555940393