Support multiple EventHub devices per InputDevice

Some physical devices contain more functions than can be addressed by
a single Linux evdev device. Examples of such devices are the Sony
DualShock 4 and Wacom Tablets, which appear as multiple evdev devices
sharing the same value for the EVIOCGUNIQ ioctl. As more instances of
such devices hit the market, apps need a way of figuring out which
InputDevices are part of the same physical device.

Per conversation with the android input team, a solution proposed to
this problem is to merge multiple EventHub devices (which have a 1:1
relation with evdev) into a single android InputDevice.

Changes:

Decouple the EventHub device id ("eventHubId") from the InputDevice
device id ("deviceId"). This requires InputDeviceContext to track the
the EventHub devices it represents. Most logic changes are inside
InputDeviceContext, so there are minimal changes to InputMappers.

Added enum value END_RESERVED_ID to represent the first available
id that can be assigned to a normal InputDevice. The android
framework assumes specific values for the virtual keyboard and
built-in hardware keyboard, so for these two cases, the "deviceId"
must match the "eventHubId."

Added "EVENTHUB_ID" constants to tests and changed where applicable.

Cherry-picked from pa/1475694.

Bug: 38511270
Test: atest inputflinger_tests libinput_tests
Change-Id: I89df085fadc1c09bc999599ac5db35c9277c4a2a