Make @RequiresPermission RetentionPolicy.CLASS.

As part of Error Prone checks, we're now placing these annotations
directly on AIDL interfaces.  However, many portions of the OS are
built using prebuilt AIDL files, where RetentionPolicy.SOURCE
annotations have been stripped away.

To retain these annotations for aiding permission enforcement
checks, this change upgrades to RetentionPolicy.CLASS.

Bug: 183626724
Test: ./build/soong/soong_ui.bash --make-mode Bluetooth RUN_ERROR_PRONE=true
Change-Id: Ic88cbf470efc1d50926fe2ff16f0e2f6cd7b1173