BatteryStats: Clamp temperature to 0

Temperature is encoded without the sign-bit, meaning
negative temperatures cause an underflow and show up
in the history as large values. We do not expect to
deal with negative temperatures for debugging/diagnostics
so just clamp the temperature to 0.

Bug: 33270828
Test: adb shell dumpsys battery set temp -10 && adb shell dumpsys batterystats --history
Change-Id: I8a92f770e4a90333eade1bb38f21d2d87bb00430