composer: Add getDisplayConnectionType

Distinguish between internal and external displays, to obviate assuming
that the first display is internal and subsequent displays are external.

Note that connector types (e.g. DSI, HDMI, DisplayPort) are not enumerated,
since that information is irrelevant for internal connections, and can be
extracted from the EDID for external connections in the few cases where it
matters, e.g. gating features like daisy chaining and content protection.

Bug: 134771872
Test: Build
Change-Id: I8a27e4ef569626620711910fcbaed5a7e12e6870