refactor: add AidlToken::Append

While parsing qualified_name, parser allocates AidlToken for each
identifier to concat them into a single AidlToken.

With Append() method, we can avoid those allocations.

Bug: n/a
Test: atest aidl_unittests
Change-Id: I41cdf880c79d57c9efd9f511075a96f3da164aea
2 files changed