OtaDexopt: Ensure float format for progress command

The progress value will be forwarded to update_engine, which expects
a "."-based float representation. Use Locale.ROOT when printing.

Bug: 37760573
Test: m
Test: adb root && adb shell cmd otadexopt prepare && \
      adb shell cmd otadexopt progress | grep 0.00
Test: Switch to German, reboot, \
      adb root && adb shell cmd otadexopt prepare && \
      adb shell cmd otadexopt progress | grep 0.00

Change-Id: I99ffdd3aefa77d77b73fb7b94accefc0f2eb3c0a