Allow configuring home button longpress duration with a flag

(cherry picked from commit 9e64d093)

Bug: 182169548
Test: tried various flag values locally on a Pixel 3 + atest SystemUITests
Merged-In: I738d86e5ef3069a03de41fa6a5bc7cdd58e3bc88
Change-Id: I738d86e5ef3069a03de41fa6a5bc7cdd58e3bc88