Fix CtsOsTestCases.android.os.cts.PowerManager_ThermalTest#testGetThermalHeadroomThresholds errors

ThermalManagerService service updateThresholds function, when initializing HeadroomThresholds,
do mask processing for SEVERE. Cause GetThermalHeadroomThresholds interface return value NAN in SEVERE patients,
so testGetThermalHeadroomThresholds CTS case fails.

The patch in updateThresholds function delete "shielding SEVERE logic", making testGetThermalHeadroomThresholds CTS test pass

Bug:361828367

Change-Id: Idb2241ae39a5aa262fb6221a3fbb08771be5387a
Signed-off-by:  <huangdezhi@hihonor.com>