Fix issues with metadata handling when format change happens

When a video format changes, the discontinuity will clear the ATSParser
queue and format. This means that AnotherPacketSource::getFormat() will
return NULL until a new format is found. The discontinuity casues the
decoder to be shutdown and NuPlayer will start scanning sources.
But since ESQueue has not yet found any format, the call to the
StreamingSource::getFormat() will return an error (BAD_VALUE) since the
convertMetaDataToMessage() fails upon NULL input.
This error code causes the scanning of sources to stop which means that
the video will be stoped.

Instead returning -EWOULDBLOCK while waiting for a new format will keep
the scanning of source.

Bug: 32007921
Test: Manual - Install SVT Play app, start live streaming
Change-Id: Ia237825361755bc63ba34b7089e7804c187fafd8