Wifi: 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: I2fde8a0207b12d9055ed0cd4b6346b678a2819bf