Move libstagefright headers to media/libstagefright/include

This is a backport of the libstagefright portion of the change to move
headers from include/ into the appropriate directories.

include/media/stagefright -> media/libstagefright/include

Adds a symlink from the old location to the new one.

Bug: 33241851
Test: mmma -j frameworks/av/media/libstagefright

Merged-In: I0ecf754a2132640ae781a3cc31428fb8c0bd1669
Change-Id: If0b10ed98e06a068985cf8d84a089e539a7cde5f