RFR - JDK-8175556 : Add support for android-arm target platform

[Gary Adams]

As a follow up to the recent openjdk arm sources moving into open repos,
this set of changes enables building android-arm vm instead of the zero vm.

When the android support was initially added to the mobile/dev repos we
required support for older android platforms. e.g. api level 19 for 
android 4.4
devices. This update raises that baseline to api level 21 for android 
5.0 and higher.

   JIRA: https://bugs.openjdk.java.net/browse/JDK-8175556
   Webrev: http://cr.openjdk.java.net/~gadams/8175556/webrev.00/webrev/

I plan to push these changes on Mon Feb 27, if there are no objections.