aapt2: Support "CANARY" as a development codename

Bug: 427283392
Flag: EXEMPT bugfix
Test: atest aapt2_tests
Test: Manual:
- Flashed device with Canary FRC build
- Used custom aapt2 with these changes in AGP
- Built and ran the app on device

Change-Id: Idd768a46c412cdc0ae66a8dcb8c30d40c3c8a706