wifi: Support 11b only mode

upgrade @1.4::ConnectionCapabilities to include legacyMode, update
corresponding 1.3 VTS test and add new 1.4 VTS test files

Bug: 163912712
Test: atest VtsHalWifiSupplicantV1_0TargetTest \
            VtsHalWifiSupplicantV1_1TargetTest \
	    VtsHalWifiSupplicantV1_2TargetTest \
	    VtsHalWifiSupplicantV1_3TargetTest \
	    VtsHalWifiSupplicantV1_4TargetTest
Test: manual test with 11b only AP to confirm 11b mode is detected and
passed to throughput predictor correctly.

Change-Id: Id4fd567401289091a5d74a770e759eee326c3873