SettingsLib: Hide NETWORK_SELECTION_DISABLED_MAX

NETWORK_SELECTION_DISABLED_MAX is a
constant, but it could change if a
new reason is added.

Instead expose a static method
getMaxNetworkSelectionDisableReason().

Bug: 146046526
Test: atest FrameworksWifiApiTests
Change-Id: I44ae60e5cece6be7469a9f3f4255c4ca5f49c4c4