wifi.proto: add SoftApReturnCode enum

SoftAp startup returns a failure mode if it does not start.  This
additional information is useful in determining where we need to add
logging and retry logic to make SoftAp mode more reliable.  The
combination of Failure + failure reason is mapped to an enum defined for
wifi metrics.

Bug: 30899122
Test: checked WifiMetricsProto.java for the enum
Test: frameworks/base/wifi/tests/runtests.sh
Test: frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Change-Id: I944e3e249d3be5738352b26a1d12139fdd50bf80