stagefright: ACodec: Resolve empty vendor parameters usage

 * On devices where vendorKeys is empty, the status was left
    as kExtensionsUnchecked and would block getVendorParameters
    on a wrong loop of empty vendor keys

 * Instead, set the status to kExtensionsNone to validate
    the check and the empty vendor keys, matching the old
    behaviour without vendor keys in getPortFormat

Change-Id: Ic3bdc9454c6af8e5d41ab901d4156ccb62281974
Signed-off-by: Adrian DC <radian.dc@gmail.com>