Add explicit method to clear clipboard.

Fix several bugs along the way:

-- Clipboard permissions weren't being revoked for related users
when a new primary clip was set.
-- checkGrantUriPermissionLocked() wasn't checking to see if an
otherwise-open provider requires permissions on specific paths.
-- When granting Uri permissions for clipboard data, we need to
include the real source UID for the grant; we no longer allow the
system UID to source grants, to avoid confused deputy problems.
-- Use the Handler passed into ClipboardManager constructor so
it lives on the right thread.

Test: cts-tradefed run commandAndExit cts-dev -m CtsContentTestCases -t android.content.cts.ClipboardManagerTest
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.AppSecurityTests
Bug: 71711122, 73797203
Change-Id: I99315035efc0c6a90471c279311294dc86766c8d