(This is a merge of http://go/wvgerrit/24738)
The change to enable 64-bit as a possible target for Widevine Android
made all the unit tests build as 64-bit on compatible platforms. Since
none of these devices support 64-bit properly yet, this is a mistake.
This patch restores unit test builds to being 32-bit only for now.
In the future, when the switch for 64-bit-supporting devices is added,
it will also switch the unit tests to 64-bit mode.
Bug: 36071236
Test: Verified unit test format with "file" command.
Change-Id: Ib4d3678181f3221b85fe6409f1d842dd15af3daa
bfe6046199