Add API to get compatibility display id

Add an API to get the compatibility display ID from CompatibilityDisplay
in the framework.

Testing Done: Compiled, built and used this API from ActivityManager and
it works.

Bug: 36071574
Change-Id: Ie4d1eb6a6befa7dbc3413519de20e2762529079d
Signed-off-by: Karthik Ravi Shankar <karthikrs@google.com>