Add @TestApi to KeyEvent.actionToString(..)

Currently, it is not possible to use KeyEvent.actionToString(..) in a
CTS test because that API is @hide. However, it would be useful to print
these actions when tests fail. Therefore, add the @TestApi annotation.

Bug: 77803694 36069459
Test: m cts-input-lib CtsHardwareTestCases (under development)
Change-Id: I2d23dbd101cef3f1c6c7a70c521a9dc219797615