NanoHTTPD: set background priority

Change-Id: I0f5c7eff74ff67d901bd4c568f8056c406dcd5a3
update support library 19+

Change-Id: I86c4cf559fdfbc7e43372e405be63976139dcc25
fix typo

Change-Id: Icc88de67dec7849cf7a9a19f20880119c2863903
add proper null checks

Change-Id: I2ef871269f0f89b3c73c1ec704e739c60a01cd45