checkapi: Allow default values for enum-type fields

We recommend to set explicit default values for enum-type fields. But
--checkapi complains about setting a default value even if the value is
zero which is compatible with ""(empty) for enum types.

For aidl_interfaces with versioned snapshots, we can't fix
-Wenum-explicit-default because --checkapi sees it as an incompatible
change to set the default value in the new while the old doesn't set
the default.

Bug: 179853398
Test: aidl_unittests
Change-Id: I11b773f7459100b23944164192379369e305783c
2 files changed