stagefright: http: Skip invalid bandwidth measurements

Measurements of 0 bytes downloaded can skew bandwidth estimation.
If 0 bytes or an invalid delay are added as a measurement, do not
add it to the bandwidth history.

Allow bandwidth estimation with a single history entry since the
total delay is now guaranteed to be non-zero for any entry.

CRs-Fixed: 808563
Change-Id: Ib68122302176f892989e37ba26e9848f0f4113d0