AImageDecoder: Document/unguard types/enums

Bug: 160984428
Test: none

From go/android-ndk-api-guidelines, these shouldn't be guarded, but they
should have an "Introduced in API ##" line. Match the guidelines.

Change-Id: I9bb51adb81782e7f6df8bd1d614d7e00a2e5556a
1 file changed