Caching result of call to ActivityManager.isRunningInTestHarness()

It's likely not a big issue as it's retrieved from SystemProperties but
because it's just for changing test behaviour and happens every time QS
is fully open - let's cache it.

Bug: 295643272
Test: Just caching value which maaaybe improves performance metrics
Change-Id: I4b0f513a7a674a18ea74be02d1137ebc5eeec2f0