runtime: Add support for dexroot-on-cache

If installd puts the dex files there, we want to read/write our oat
files out of there, too

Change-Id: I7da528622d7c855431dd99b20ff54da791bc1681
runtime: dexroot-on-cache: obey dalvik.vm.dexopt-data-only

Change-Id: Ie037706b1004e014c20f81c8fcdc219c46da57c7
runtime: Invert the default for dexopt-on-cache

The growth of /system apps and total /data space are making this
device-breaking (insufficient space to dexopt everything to
/cache), pointless (/data has more than enough room to hold
the cache), and in some cases, both.
The default behavior is now the same as mainline Android's: dexopt
everything to /data. Set dalvik.vm.dexopt-data-only=0 to change
that and dexopt /system apps to /cache. If you do that, make sure
your /cache can accomodate double the size of the dex classes in
system, since that has been observed during dex2oat

Change-Id: I8c1debee6e16a6fb162e42a7c9a47ec076eb69df