Add a comment for the LegacyGlobalActions broadcast receiver fix

The bug is already fixed in master branch by registering the broadcast
receiver with UserHandle.ALL, which is an uncommon way to register
a broadcast receiver. This CL adds a comment to explain it.

Bug: 182842046
Test: atest CtsAccessibilityServiceTestCases
Change-Id: I10405769ba6426c7bbcd31e09e1cd4c99585665b