Update API txt following a metalava fix

The txt was previously referencing VcnGatewayConnectionConfig.MIN_MTU_V6
which isn't part of the API. The API txt should be complete on its own,
so inline the value instead.

Similar case for appsearch annotations referencing non-API values.

Bug: 188527836
Test: m checkapi
Change-Id: Ideb34ab78c9a8fc4d589e58e810316b6f36b7a43