Check if the window is partially obscured for slippery enters

Currently, we only check whether a window is partially obscured during
the initial tap down. However, there is another use case: slippery
enter.

During a slippery enter, the touch down is generated into the
slipped-into window, and touch cancel is generated for the slipped-from
window. The window receiving the slippery enter does not need to have
any flags.

Until we figure out whether we can restrict the usage of this flag to
system components, add this check as an intermediate fix.

Bug: 157929241
Test: atest FlagSlipperyTest
Test: atest inputflinger_tests
Change-Id: I93d9681479f41244ffed4b1f88cceb69be71adf2
Merged-In: I93d9681479f41244ffed4b1f88cceb69be71adf2
(cherry picked from commit 870ececa)