ART: Change ArtMethod::NumArgRegisters() signature.

Take "const char*" instead of StringPiece. Avoid calling
strlen() in all callers, rely on explicit checking for
the end of the string.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 123750182
Change-Id: I189841e4ae7dcc5950616d5e5a590987618146cb