libstagefright: Fix ANR issue in HTTP Streaming

The root cause of the ANRs during this usecase is the
blocking nature of the read calls on the network data
source due to which application thread gets blocked on
some lock in framework.

Fixes ANR during seek/pause during HTTP streaming
* Non-blocking read implmentation for NuCachedSource2
* Other framework related changes to handle underruns

Change-Id: Ief280b7e23727606746e1ca2bf4caa4a3044b7bc