Duration format for tunable settings

For JobScheduler, DeviceIdle and AppStandby constants, allow
using a more compact format than milliseconds,
which are a PITA to calculate.

So instead of 18640000000... whatever, you can
use PT2H (for 2 hours), or P2D (for 2 days), etc.
Uses Duration.parse() to do the parsing. See Duration
for format.

Test: adb shell settings put global app_standby_constants
      screen_thresholds=0/PT2H/PT12H/P2D
Fixes: 71554131
Change-Id: I5141854ec7df6de266725a67f1f3e2a6e0b4c1c1