Treat all samples as sync frames in AviExtractor when appropriate

Update ChunkReader in AviExtractor to check
MimeTypes.allSamplesAreSyncSamples(mimeType, codec). This ensures that
for formats where all samples are guaranteed sync frames (such as audio
tracks), samples are marked with BUFFER_FLAG_KEY_FRAME even if the index
table omits the keyframe flag.

Also add missing test.parameter.injector dependency to lib-decoder-av1
in build.gradle.kts to fix Gradle test compilation.

PiperOrigin-RevId: 941011394