Turn on rounded corners on PIP with a flag

Bug: 163076219
Test: manual

Use the flag "debug.sf.enable_hole_punch_pip" (also used by
I56e2a6debce6ede4bebfcbd32bffa01c20461542 to activate using a hole punch
to render the rounded corners) to turn on rounded corners even if
config_pipEnableRoundCorner is set to false.

This will allow testing the feature until we can turn it on.

Change-Id: I2719f8ac151895874ab22e4cd7e64c9e1dc9eac8