Revert "Remove InputMethodManagerService#mCaller"

This reverts commit 7c46dc83efe879304dde3cadd105d0b96547e73d.

Reason for revert: Droidfood blocking bug: 216588046

Bug: 192412909
Bug: 216588046
Change-Id: I12902c08d21d851a13ef7319f87e5dc784182228
(cherry picked from commit cc0498759b0e752d73d0551afa33ed9675d6e445)
Merged-In:I12902c08d21d851a13ef7319f87e5dc784182228
1 file changed