SF: update mInputFlinger on main thread

mInputFlinger can be accessed by bootFinished() and updateInputFlinger().
These function can run in different thread at the same time, but
mInputFlinger was not protected.

update mInputFlinger on main thread

Fixes: 157871763
Fixes: 147009853
Fixes: 169256435
Test: adb shell stop / start
Test: boot ok
Test: Race was only reproducible in monkey tests
Change-Id: I4d87e90793a88a646aaa1ae5806f118f1ae51e30
Merged-In: I4d87e90793a88a646aaa1ae5806f118f1ae51e30
(cherry picked from commit 58fbf9fb)