Add host prebuilts of guava, jsr305, and truth

Add guava-20.0, jsr305-3.0.1, and truth-0.28 prebuilts needed by
external/desugar.

Also remove guava-15.0, it's not used any more.

Test: builds
Change-Id: I8c13b4fb90bc66ddd659f898993faef5bef59838