checkapi works with imports

Without imports checkapi can't properly work with references to imported
types. We had to work around it till now because TypeSpecifiers and
ConstantReferences are not resolved,

Now, checkapi works with imports including:
- include_dirs
- "current" api dumps of imported modules

Bug: 188878102
Test: m aidl_unittests
Change-Id: If0bed7471ac7cbb944fa6e136b553dac0ce93612
6 files changed