Enable TETHER_ENABLE_SYNC_SM by default

The tether_enable_sync_sm flag has been successfully rolled out
to public stage on Pixel devices without any observed
regressions. As a result, this flag should be enabled by default
with a kill switch, allowing the feature to be deployed to all
production devices.

Test: atest TetheringTests CtsTetheringTest
Change-Id: I2f0f55c1586e1f4987b28488082c4c1f4cae0f5e
3 files changed