[RenderScript] Set RS_PREBUILT_LIBPATH for 64bit targets.

Bug: 22796994
Bug: 22693954

This change fixes the RS_PREBUILT_LIBPATH when used with 64-bit target
devices. It also fixes an issue where the target RenderScript API is
below 21 for 64-bit compilation. In those cases, we should always
upconvert it to 21 (since that is the first available 64-bit release).

Change-Id: I27e9d0bd5c01bac0b3b28e9002333264a55d628f