Use SysuiTestCase#runShellCommand.

This (a) makes the test run rather than fail for trying to get the
global instrumentation.
(b) May deflake the test (once it is passing) by waiting for shell
commands to complete before moving on.

Bug: 176891566
Bug: 249756667
Test: com.android.systemui.globalactions.GlobalActionsImeTest
Change-Id: I7dbf1b5443a501bb71dafc7bbd89b43b2a09a0b2