Prototype for Sharesheet direct share row append mechanism

Use this cmd to enable the feature:
adb shell device_config put systemui append_direct_share_enabled true

Use this cmd to adjust timeout threshold (in millisecond):
adb shell device_config put systemui share_sheet_direct_share_timeout 15000

Bug: 151112858
Test: manually tested both prod flow and prototype flow on phones.

Change-Id: I328ecefc9dffad40ec412c033da54e0443f8889a