Don't allow MemoryFiles of negative length.

Prevents us from converting a (signed) jint into an
(unsigned) size_t and having horrible things happen.

Change-Id: I0f04e2eb9852ae7fc49b435fd0974f56e86751a4
Set some GC parameters only for dalvik

Commit https://github.com/CyanogenMod/android_frameworks_base/commit/8f566390bceee9f01b1721c4d00b1b246d26be16
fixed parsing of certain GC values. Before the result was "" and so the
if statement was always false, never executing the setTargetHeap* calls.

Now they are executed but ART currently doesn't support setting
heapminfree and heapconcurrentstart resulting in a UnsatisfiedLinkError
Exception.

Check for the used runtime first and only execute the methods for Dalvik.

Change-Id: I09e60802685acbec04598646e9f7700f33b53b4c
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>

Patchset: 3
http://review.cyanogenmod.org/#/c/62128/