ANDROID: PM / EM: Document the support for legacy (deprecated) EM

Android 4.19 has a compatibility driver enabling to load legacy EMs (now
DEPRECATED) into the new framework.

Although it is NOT RECOMMENDED to use this feature, it *is* there, so we
should at least document it. It's also an opportunity to make clear it's
deprecated, and to point at the alternative solutions.

Bug: 120440300
Change-Id: I1ec43208c2727b878d31c508153e2a0a0e7fa55b
Signed-off-by: Quentin Perret <quentin.perret@arm.com>