Use Runtime.gc instead of System.gc for DDMS.

System.gc doesn't always do a GC unless to attempt to run the
finalizers after you call System.gc.

Bug: 14325353
Change-Id: Iebed7fad5576d610cea13a86e7d3d46652c67478
Merge "Use Runtime.gc instead of System.gc for DDMS."
Track libcore.os' move to android.system.

(This is partial, but should cover everything in AOSP master except
for the zygote.)

Change-Id: I1042c99245765746a744c44e714095cb2c6cb75d
Merge "Track libcore.os' move to android.system."
Fix build.

Change-Id: I0bd6e1aac19276bf664accd8d38f86d2cc7450cc
Merge "Fix build."
auto-merger: merge in from aosp/master