Introduce the flags of the insets source

This enables us to control the behavior of a specific insets source. The
first flag added in this CL is FLAG_SUPPRESS_SCRIM. If the insets source
owner sets this flag, the system won't draw a semi-transparent scrim at
the insets area.

Fix: 197615177
Test: Open a work-profile app with gesture nav bar. See if there is no
      scrim at the nav bar area.
Change-Id: I32449805f47ba6c44e6eec7b512a6b1a0833371e