Fix clang-tidy warnings in aapt and aapt2.

* Add explicit keyword to conversion constructors.
* Add NOLINT(implicit) comments for implicit conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
* Use const reference type to avoid unnecessary copy.
Bug: 30413862
Test: build with WITH_TIDY=1

Change-Id: Id6d21961f313a1ad92b15a37fdaa5be9e8ab48e1
Merged-In: Id6d21961f313a1ad92b15a37fdaa5be9e8ab48e1