Remove org.apache.http.legacy from bootclasspath

Updated data/etc/platform.xml to make the org.apache.http.legacy library
usable by applications. The runtime location of the
org.apache.http.legacy library is
/system/framework/org.apache.http.legacy.boot.jar not
/system/framework/org.apache.http.legacy.jar.

Stop removing org.apache.http.legacy from the required and optional
shared library lists and instead add it to the required list if it is
not present in either and the package is targeted at SDK
version <= O_MR1.

Bug: 18027885
Test: make FrameworksCoreTests, install and run
Change-Id: I686e3c20f5860e58825e1b88f220f9b8c335849c