Support aquery depsets with depth

In Bazel aquery responses, Bazel represents action inputs by preserving
the form of the depset data structure which contains them.
(https://docs.bazel.build/versions/master/skylark/lib/depset.html)
Thus, Soong's aquery handler must appropriately "flatten" this
data structure in cases where the depset consists of multiple levels.

Test: m nothing
Test: lunch aosp_flame && USE_BAZEL_ANALYSIS=1 m libc
Change-Id: I2ebacac1deea7538eb34ad1975594caae71091a2