Support build rules which generate .o files.

webviewchromium has some build rules which use a custom tool to create
.o files from other input (i.e. they are not prebuilt and so can't be
included in LOCAL_PREBUILT_OBJ_FILES). Support adding .o files to
LOCAL_GENERATED_SOURCES and doing the right thing with them (including
them in the static/dynamic library or executable being built).

Bug: 7714333

Change-Id: I3b1d29eeff30aebeafe33398f9bef2eb6972d997