Retry: Allow modules to override --error-when-new UnflaggedApi

Some `java_sdk_library` and `droidstubs` modules already specify
`--error UnflaggedApi` but they are currently ignored because they are
added before the `--error-when-new UnflaggedApi` automatically added by
Soong and Metalava uses the last setting. That means adding
`--error-when-new UnflaggedApi` in change https://r.android.com/3248013
actually reduced the severity of the issue for those modules making it
more likely that they would be missed.

This change only adds the `--error-when-new UnflaggedApi` if it does
not already appear in the metalava command.

Bug: 361582214
Test: m checkapi
Change-Id: I7228bf9794b1c7e27d7acf7a461956b30f3335f5