Revert "Ensure synchronized access of JNI callback object"

This was supposed to fix screen-on buffer underruns with angler, but it
did not solve the issueso I'd rather revert this now than it causing
issues down the line.

This reverts commit 2c9c5455.

Change-Id: I2bc62df648f75cc6ba8e64db04cdc378ecc6f23d