Use RRO for the "Cancel" button instead of FEATURE_PC

FEATURE_PC is not a reliable way to identify a certain type of device.
Instead let's be clear that this functionality (showing a "Cancel"
button on all pickers) can be optionally enabled via an RRO config
value.

Bug: 414694077
Test: atest com.android.documentsui.PickActivityTest
Flag: com.android.documentsui.flags.use_material3

Change-Id: I24241dc15e174eb4bd3066ffaf96e3ea9b92e055