should return correct length and use shorter poll time for ad2p data

the default poll timeout(100ms) is too long for a2dp data read,
because the a2dp timer pertiod to read data is 20ms. The poll timeout
should be far less than a2dp timer pertiod. also when poll timeout happen
for read, we should return the data length which we already read instead of 0.

bug:8252054
Change-Id: Ifa8587c6b7d127cbbbff76d9e20b630303b6379f