Allow patch_module attribute in target: { android: { ... } } block.

patch_module is currently only used for non-host targets, although
there's a slim chance that there might be use cases on host in
future.

Currently, the only use case is patch_module: "java.base" in
device targets. To avoid erroneously carrying the patch_module
attribute over into the host version of the target, this CL
lets them place it into the target: { android: { ... } } block.

Test: Treehugger
Test: EXPERIMENTAL_USE_OPENJDK9=true make android-icu4j-tests
Bug: 115604102

Change-Id: If025d503d9c5681748a502a1d7433e8dbec3cac1