Allow IME to register callbacks with negative priorities.

It's perfectly valid to register system callbacks (which have negative
priority) from the IME process from framework code, once it is opted in.

We should still reject negative priority for registration through public APIs though. Will follow up separately in b/262930093.

Test: atest android.view.inputmethod.cts.KeyboardVisibilityControlTest#testHideImeAfterBackPressed_legacyAppMigratedIme
Fixes: 262727745
Change-Id: Ie13c738d0d4a10f4265fbf116ff7b4206d3bfc62