libstagefright: Fix crash in convertMetaDataToMessage

- The ABuffer used for the Message has a preset value of 1024, if
  flattening the meta data exceeds this value, a check fails hence
  the crash.
- This change creates a new ABuffer if the buffer size would exceed
  the buffer capacity.

CRs-Fixed: 857850

Change-Id: Iaa7374a4734a49db257a3f102a88412fde672260