[Media ML] Support customization for long press implementation

Add code for checking whether events for certain key codes have been
customized by vendors.
Also track KeyEvents so that MediaSessionService can determine
whether a series of KeyEvents can be considered a long press.

Will add code for distinguishing single/double/triple clicks in the
next CL.

Bug: 149260441
Test: manually
Change-Id: I283a5ba36c2db0b69fd64d0127d40b11fe1c6679