Opp: Replace "direction" usages with a constant

There are many places that uses a string value "direction" to get/set
an intent extra.

This CL makes the code more readable by defining a new constant for
intent extra key - but with a different value.

Bug: 318610752
Test: atest BluetoothOppReceiverTest BluetoothOppTransferHistoryTest
Flag: EXEMPT, string value change
Change-Id: Ic9bc8609240f0d09ad2b47186f3918b83c3c1f21