SF: Refactor HWComposer::onHotplug()

Currently HWComposer::onHotplug() contains logic that is
executed when hotplug disconnect or connect event occurs.
However reading what exactly is done during e.g. hotplug
connect is hard, because the logic is spread between two
if statements and the function HWComposer::onHotplugConnect().
This change moves all hotplug connect logic to
HWComposer::onHotplugConnect() and all disconnect logic
to a new function HWComposer::onHotplugDisconnect().

Additionally this CLs refactors onHotplugConnect() which
is currently responsible for multiple concerns, which is
making the code hard to understand and reuse.

 - checking if the event should be ignored is moved to
   a new function shouldIgnoreHotplugConnect()
 - creating the necessary state for the new display is
   moved to a new function allocatePhysicalDisplay()

Some of the newly created functions will be directly reused
in following CLs.

Bug: 143451809
Test: m surfaceflinger
Test: atest libsurfaceflinger_unittest
Change-Id: Ib4dae2ce81e8cac176a56b6de787a4f76d01c050