Camera: Address EFV latency API feedback

Update javadocs and interface signatures of
getRealtimeStillCaptureLatency(),
getSupportedPostviewOutputResolutions() and
startMultiFrameCapture() according to the
API council feedback.

Flag: com.android.internal.camera.flags.efv_capture_latency
Bug: 427953808
Bug: 297083874
API-Coverage-Bug: 433563889
Test: Successful build
Change-Id: I657eb60617350061155f056c25113e561fc12f5e