Add "required_parameters" to Annotation params

Some annotations must have a set of parameters. To model this
requirements, the schema for annotations has "required_parameters" now.

For example, @Backing for enum types should have "type" parameter even
though the annotation itself is optional.

AidlAnnotation::CheckValid() checks if there's no missing required
parameters.

Bug: none
Test: aidl_unittests
Change-Id: I50d82a1576f62bfdc2f220b66b7475cf74198e53
3 files changed