audio: Put stronger rules on vendor extension enums

The namespace part has been made mandatory. The namespace
must be at least 3 characters.

Bug: 176144684
Test: atest android.hardware.audio.common@7.0-util_tests
Test: atest android.hardware.audio@7.0-util_tests
Test: atest android.hardware.audio.effect@7.0-util_tests
Change-Id: If8578dfab80b51d9c30042e99bfbf70f40598afa