libstagefright: Update handling of SBR explict siganlling

In case of SBR explict signalling in the bitstream, the core
AAC sample rate is overwritten by extension sample rate.
HW AAC decoder relies on core sample rate to parse the
bitstream when supplied in raw format.
Do not overwrite core sample rate but store it as separate
field in the meta data.

CRs-fixed: 572242

Change-Id: I5c70cfcc40a44bff9f9da4082c6b4dccd7c6788e