java_sdk_library: Require xml permission file only if it is generated

Eliminates invalid LOCAL_REQUIRED_MODULES lines in the generated
Android.mk entries.

Bug: 7456955
Test: TH artifact noop
Test: Add unittest to java/androidmk_test.go
Test: m nothing and verify build rule doesn't contain invalid REQUIRED
Change-Id: Ic8d7e6d08181f6a24fd2aa86479bb9303cfd9be8