rust: Only pass "-lstatic" for rlibs.

Passing '-lstatic' for dylib static dependencies results in some
unexpected bloat as the static libraries are included as
whole-archives.

For now, limit this to rlibs while we investigate.

Bug: 175886967
Bug: 179122837
Test: cd system/bt/; mma
Test: Checked size of dylibs.
Change-Id: Id8328e95771e2ca8503bc8df35248b1c39cabc8e