Rename getters and setters on ImageDecoder

Bug: 76448408
Test: Ib8782ff10072c81a5ed2a1031a70475fffee7ccf

- Use "is" prefix for booleans instead of "get"
- Reverse "require" and "unpremultiplied" for a more natural sound
- Add "Required" to "Mutable" methods
- Add "Enabled" to "DecodeAsAlphaMask" methods

Change-Id: I644ddccd37898d89609e4534ece4ea70f74587c4