Have getSupportedModemCount always rely on system properties.

Earlier we disabled the reboot free feature and always have
getSupportedModemCount return the same value with getActiveModemCount.
Now that we've enbaled reboot free switch, we will return correct value
based on system property.

Bug: 142514392
Test: manual - change PROPERTY_REBOOT_REQUIRED_ON_MODEM_CHANGE and test
what getSupportedModemCount returns.
Change-Id: I34e172c747dd52aa498037e64b758393793fddba
Merged-In: I34e172c747dd52aa498037e64b758393793fddba