GenericSource: Synchronize access to dataSource

DataSource and HTTPSource can be accessed/modified by
GenericSource's looper and Client's thread which can
lead to race conditions when copying the sp<>.
Add a Mutex lock to synchronize such accesses and avoid
race conditions.

CRs-Fixed: 906899
Change-Id: I2fb4b4a7079e638e151f4fe67a780007a4011652