httplive: refactor for HLS customization

List of changes that are squashed:-

Extend NuPlayer setDataSource for streaming.
Change-Id: Ic87c3744bf905eb8742863951b809e38d0a60339

httplive: Add default implementation in HLS for custom changes
Change-Id: I5e2bc7a42e0d4a16d5b5ea5f2ec3d78a7d019ad3

Stagefright: Extensions for HEVC in ATSParser
Change-Id: I05d623fbf5a4e2b260f68c5e591ac0f123ef2f37

Change access modifiers and add overridable methods in HLS stack.

Change-Id: Iae8e77246cc6643735af18617717fba713d0038c